Solvent-dependent intramolecular charge transfer dual fluorescence of p-dimethylaminobenzanilide bearing steric ortho,ortho-dimethyl substituents at amido aniline

Article abstract of DOI:10.1039/c1pp05208j

Intramolecular charge transfer (ICT) dual fluorescence was observed in various organic solvents with p-dimethylaminobenzanilide (DMBA) derivatives bearing ortho-methyl (DMOMBA) and ortho,ortho-dimethyl (DMDMBA) substituents at amido aniline moiety. Ab initio calculation and absorption spectral data indicated that high steric hindrance was introduced by the ortho,ortho-dimethyl substitutions. It was found that, with DMDMBA, the CT emission initially shifted to the red with increasing solvent polarity from cyclohexane (CHX, 480 nm) to diethyl ether (DEE, 520 nm), similar to those of DMBA derivatives with the ortho-, meta- or para-methyl substitutions at amido aniline moiety. However, there is a characteristic blue-shift of the long wavelength emission between DEE and tetrahydrofuran (THF, 424 nm) then a bathochromic shift again in highly polar solvent acetonitrile (ACN, 484 nm). The unusual solvent-dependent CT emission was ascribed to two competitive CT channels. One is benzanilide (BA)-like CT, whose CT reaction occurs from amido aniline to benzoyl moiety in nonpolar solvent CHX and DEE; the other one is p-dimethylaminobenzamide (DMABA)-like, whose CT reaction occurs from dimethylamino to benzanilide moiety in highly polar solvent THF and ACN. These findings revealed the steric effect plays an important role in the ICT process, which may alter the properties of the electron donor and/or acceptor, but also change the reaction potential.

Full text of DOI:10.1039/c1pp05208j

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PAPER
Solvent-dependent intramolecular charge transfer dual fluorescence of
p-dimethylaminobenzanilide bearing steric ortho,ortho-dimethyl substituents
at amido aniline
Xuan Zhang*a and Yun-Bao Jiang*b
Received 30th June 2011, Accepted 5th August 2011
DOI: 10.1039/c1pp05208j
Intramolecular charge transfer (ICT) dual fluorescence was observed in various organic solvents with
p-dimethylaminobenzanilide (DMBA) derivatives bearing ortho-methyl (DMOMBA) and
ortho,ortho-dimethyl (DMDMBA) substituents at amido aniline moiety. Ab initio calculation and
absorption spectral data indicated that high steric hindrance was introduced by the
ortho,ortho-dimethyl substitutions. It was found that, with DMDMBA, the CT emission initially shifted
to the red with increasing solvent polarity from cyclohexane (CHX, 480 nm) to diethyl ether (DEE, 520
nm), similar to those of DMBA derivatives with the ortho-, meta- or para-methyl substitutions at amido
aniline moiety. However, there is a characteristic blue-shift of the long wavelength emission between
DEE and tetrahydrofuran (THF, 424 nm) then a bathochromic shift again in highly polar solvent
acetonitrile (ACN, 484 nm). The unusual solvent-dependent CT emission was ascribed to two
competitive CT channels. One is benzanilide (BA)-like CT, whose CT reaction occurs from amido
aniline to benzoyl moiety in nonpolar solvent CHX and DEE; the other one is
p-dimethylaminobenzamide (DMABA)-like, whose CT reaction occurs from dimethylamino to
benzanilide moiety in highly polar solvent THF and ACN. These findings revealed the steric effect plays
an important role in the ICT process, which may alter the properties of the electron donor and/or
acceptor, but also change the reaction potential.
investigations, the possible steric effect was avoided by introducing
substitution only at the para- or meta-position of the phenol
1. Introduction
moieties in the phenyl p-dimethylaminobenzoates,14 or the amido
aniline moieties in p-dimethylaminobenzanilides (DMBA, X = H,
Scheme 1),17 to modify the electron donor/acceptor. The ortho-
substituent effect on CT process, however, remained unclear and
requires some further investigation.
Intramolecular charge transfer (ICT) dual fluorescence of p-
dimethylaminobenzonitrile (DMABN) and related derivatives has
drawn much experimental and theoretical attention.1–12 Both the
electronic and steric factors were considered to be important for
the occurrence of the charge transfer reaction.2,3 Rettig et al.13
have examined the electronic factors of the electron acceptor
on the CT emission of p-dimethylaminobenzamide (DMABA,
Scheme 1) by diminishing the electron-acceptor character, by
varying the amide group from –C(O)NH2 through –C(O)N(CH3)2
to –C(O)N(C2H5)2. Although a systematic blue shift of the CT
emission and the decrease of the emission intensity ratio between
CT and locally excited (LE) states were observed, it was found
that a considerable steric effect was also introduced at the amide
moiety that may also contribute to the change of CT emission.13
Recently, we employed an alternative strategy that successfully
showed the important role that the electronic factor of the
electron donor/acceptor played in the CT process.14–18 In these
aCollege of Chemistry, Chemical Engineering and Biotechnology, Donghua
University, Shanghai, 201620, China. E-mail: xzhang@dhu.edu.cn
Scheme 1 Molecular structures of investigated compounds. X in DMBA
are H, p-CH3 and m-CH3, respectively. The numbers were used to define
the dihedral angle.
bDepartment of Chemistry and the MOE Key Laboratory of Analytical
Sciences, Xiamen University, Xiamen, 361005, China. E-mail: ybjiang@
xmu.edu.cn
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Table 1 Dihedral angles, bond lengths and dipole moments in the ground statea
Dihedral angels, ◦
Bond lengths, A
b
˚
Compounds
q1
q2
q3
q4
C(O)–N
C(Ph)–N
Dipole moment, D
DMBA
X = H
X = p-CH3
X = m-CH3
DMOMBA
DMDMBA
1.3
1.3
1.6
0.0
58.9
2.3
2.5
2.1
2.5
42.8
20.1
20.2
20.1
19.6
20.6
17.4
17.5
17.5
16.0
18.5
1.372
1.370
1.371
1.393
1.395
1.419
1.420
1.419
1.423
1.437
5.39
5.38
5.14
5.82
5.50
a Calculated using Gaussian 9824 and the molecular structures are shown in Scheme 1. b Dihedral angels defined in Scheme 1, q1, ∠C6¢–C1¢–N9–C7; q2,
∠C2¢–C1¢–N9–H10; q3, ∠C2–C1–C7–N9 and q4, ∠C6–C1–C7–O8.
With DMBA (X = H, Scheme 1), a compound combining
the competitive CT reactions of both DMABA13 and ben-
zanilide (BA),15–22 it was found that, with increasing the electron-
withdrawing ability of the para- or meta-substituent to the amido
anilino moiety, the CT reaction initially occurred from the
amido aniline moiety to the rest of the molecule (BA-like), and
tended to occur from dimethylamino group to benzanilide moiety
(DMABA-like).17 The peculiar CT character of DMBAderivatives
prompted us to select it as a model system to further examine the
ortho-methyl substitution effect on the CT reaction.
and emission slits of 5 nm. The excitation wavelength was 305 nm.
Fluorescence quantum yields (UF) were measured using quinine
sulfate as a standard (0.546 in 1 N H2SO4).23 Ab initio calculations
at Hartree–Fock (HF) self-consistent field level were carried out
using Gaussian 98.24 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
sample concentrations of ca. 10-5 M.
3. Results and discussion
We report herein a description of the steady-state absorption
and fluorescence spectra of DMBA derivatives with ortho-methyl
and ortho,ortho-dimethyl substitutions to the amido anilino
moiety (DMOMBA and DMDMBA, Scheme 1). Comparison
of ab initio calculations and absorption spectral data indicates
that the ortho,ortho-dimethyl substitution introduces a significant
steric change in the structure of DMDMBA derivative, and as
a consequence the CT fluorescence was found to be solvent
dependent, which was BA-like in nonpolar solvents such as
cyclohexane (CHX) and diethyl ether (DEE), but DMABA-like
in polar solvents such as tetrahydrofuran (THF) and acetonitrile
(ACN). The reason of the influence of the steric hindrance on the
solvent-dependent CT emission behavior is discussed in the paper.
3.1 Optimized geometry of DMBA derivatives
To evaluate the measure of steric hindrance introduced by the
ortho-methyl substitutions at amido anilino moiety, ab initio calcu-
lations were performed on DMOMBA and DMDMBA (Scheme
1) and parts of the structure parameters were presented in Table 1,
in which the corresponding values of DMBA and its para- or meta-
methyl substituted derivatives were also listed for comparison. As
noted from Table 1, the ground state conformations of DMBA
and its para- or meta-methyl substituted derivatives, as well as
DMOMBA were almost planar (q1 < 2◦, q2 < 3◦), whereas
DMDMBA was very twisted at the amido aniline moiety (q1 =
58.9◦, q2 = 42.8◦), even the amide benzoyl dihedral angles q3
and q4 were almost the same in all of the DMBA derivatives
(Scheme 1). While the slight variation was found in the ground
state dipole moment, both the bond lengths of amide C(O)–N and
C(Ph)–N bonds of DMDMBA were obviously longer than those
of other DMBA derivatives investigated (Table 1). Therefore, it
is reasonable to conclude that in the case of DMDMBA, the
ortho,ortho-dimethyl substitution introduces a large increase in
the angle of plane of the amido anilino moiety compared with the
plane of the DMBA subgroup, as it is otherwise expected.
2. Materials and methods
DMOMBA and DMDMBA were synthesized by heating p-
dimethylaminobenzoic acid and the corresponding substituted
aniline in the presence of POCl3 as described previously.17
4-Dimethylamino-2¢-methylbenzanilide (DMOMBA). IR
1
(KBr, cm-1), 3294, 3047, 2919, 1631, 1604. H NMR (500 MHz,
DMSO-d6), d (ppm) 2.22 (s, 3H), 2.99 (s, 6H), 6.75 (d, 2H, J = 9
Hz), 7.13 (m, 1H), 7.19 (m, 1H), 7.24 (d, 1H, J = 7 Hz), 7.32 (d,
1H, 9 Hz), 7.86 (d, 2H, 9 Hz), 9.47 (s, 1H).
3.2 Absorption spectra of DMBA derivatives
4-Dimethylamino-2¢,6¢-dimethylbenzanilide (DMDMBA). IR
1
(KBr, cm-1), 3288, 3037, 2920, 1631, 1609. H NMR (500 MHz,
Fig. 1 showed the absorption spectra of DMBA, DMOMBA,
DMDMBA and DMABA in cyclohexane (CHX) and acetonitrile
(ACN). The absorption spectra of DMOMBA in CHX were found
to be only slightly shifted to the blue by ca. 3 nm than that
of DMBA.17 With DMDMBA, however, the absorption spectra
were significantly blue-shifted (ca. 12 nm) and more similar to
that of DMABA (Fig. 1a), revealing that high steric hindrance
was indeed introduced by ortho,ortho-dimethyl substitution.25,26
In all cases, an obvious shoulder can be seen at ca. 300 nm,
which is scarcely affected by substitution and readily ascribed
DMSO-d6), d (ppm) 2.16 (s, 6H), 2.99 (s, 6H), 6.76 (d, 2H, J = 8.5
Hz), 7.10 (s, 3H), 7.88 (d, 2H, J = 9 Hz), 9.38 (s, 1H).
IR data were taken on a Nicolet FT-IR 360 spectrophotometer
1
with a KBr pellet and H NMR data were acquired in DMSO-
d6 on a Varian Unity+ 500 MHz NMR spectrometer using TMS
as the internal standard. The absorption spectra were recorded
on Varian Cary 300 spectrophotometer coupled with a 1 cm
quartz cuvette. Corrected fluorescence spectra were taken on a
Hitachi F-4500 fluorescence spectrophotometer using excitation
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Table 2 Spectroscopic data for DMOMBA and DMDMBA in organic solvents
Absorption
Fluorescence
LE/lCT, nm
b
Compounds
DMOMBA
Solventa
f –f ¢
l
max, nm
e, 104 M-1 cm-1
l
UF
CHX
DEE
THF
ACN
CHX
DEE
THF
ACN
0.000
0.161
0.208
0.306
0.000
0.161
0.208
0.306
304
302
305
309
295
293
296
302
2.50
2.54
2.54
2.39
2.89
2.86
3.47
3.34
343/460
346/508
354/518
370/498
336/480
344/520
352/424
364/484
0.0011
0.0070
0.0043
0.0059
0.0015
0.0036
0.0059
0.0158
DMDMBA
a Solvent: CHX, cyclohexane; DEE, diethyl ether; THF, tetrahydrofuran; and ACN, acetonitrile. b Taken from ref. 27.
affected the absorption spectrum of DMBA, in which only the
substituent electronic effect was considered to be involved.17 We
could therefore conclude that the steric hindrance introduced
by ortho,ortho-dimethyl substitution of DMDMBA, affects the
energy of the electronic transitions, in good agreement with the ab
initio calculations (Table 1).
3.3 Fluorescence spectra of DMBA derivatives
The fluorescence spectra of DMOMBA and DMDMBA were
presented in Fig. 2. From CHX to DEE, the dual fluorescence
was observed for DMOMBA and DMDMBA, which is very
similar to those of the para- or meta-methyl substituted DMBA
derivatives.17 With increasing solvent polarity from CHX to DEE,
while the short-wavelength emission underwent a minor variation,
the long-wavelength fluorescence remarkably shifted to the red
(Fig. 2 and Table 2), indicating the charge transfer nature of the
emissive state. The dual fluorescence was hence readily attributed
to the LE and CT states, respectively. As suggested for other
DMBA derivatives,17 the CT reaction probably occurs from amido
aniline to benzoyl moiety (BA-like).15–22 However, the CT emission
of DMDMBA shows a surprising blue-shift in THF (a solvent
of medium polarity) compared with the less polar DEE, but it
shifted to the red again in highly polar solvent ACN (Fig. 2a and
Table 2). This could be explained as BA-like CT reaction mixing
with the DMABA-like CT reaction, since it is known that in the
same solvent the DMABA-like CT emission locates at a shorter
wavelength than the BA-like CT emission.15,17
To trace the detailed change of solvent-dependent CT emission,
the fluorescence spectra of DMDMBA were recorded in DEE-
ACN binary solvents and the normalized emission spectra are
shown in Fig. 3. With increasing solvent polarity by increasing the
ACN portion in the binary solvents, it could be observed clearly
that the BA-like CT emission around 530 nm was quenched, but
a new emission gradually emerged at shorter wavelength (ca. 430
nm) and red-shifted strongly, which indicates the CT nature of
the new emissive state. Comparing the CT emission behavior of
DMDMBA with those of DMOMBA and para- or meta-methyl
substituted DMBA derivatives (Scheme 1), the fluorescence of
DMDMBA in THF and ACN can be readily ascribed to the
DMABA-like CT emission.17 This was supported by the fact that
the emission behavior of DMDMBA in ACN was very similar to
that of DMABA in the same solvent (Fig. 2b).13
Fig.
1 Normalized absorption spectra of investigated compounds
(Scheme 1) in CHX, cyclohexane (a) and ACN, acetonitrile (b).
to 1Lb state transition, while the main absorption assigned to 1La
state transition that is sensitive to the substitution as observed
previously in donor–acceptor-substituted aniline derivatives.13 In
the highly polar solvent ACN, the absorption spectra of all
DMBA derivatives only slightly shifted to the red than those in
CHX, reflecting the small dipole moment of the ground state,
meanwhile the absorption bands were well separated with different
substitution (Fig. 1b and Table 2). It has been shown that para-
and meta-methyl substitution at the amido anilino moiety scarcely
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3.4 Solvent-dependent ICT dual fluorescence of DMDMBA
Fig. 4 shows the variations of the CT to LE intensity ratio and CT
emission energy of DMBA and its para-, meta-, ortho-methyl and
ortho,ortho-dimethyl substituted derivatives with solvent polarity
indexed by f-f ¢.27 It can be noted that the ratios of CT to LE
intensities of DMBA and its para- and meta-methyl substituted
derivatives initially increased with increasing solvent polarity
(from CHX to DEE), then decreased with further increasing
polarity through THF to ACN (Fig. 4). Even the similar variation
profile was observed in ortho-methyl substituted DMOMBA,
however, a very different profile was found in highly steric
ortho,ortho-dimethyl DMDMBA (Fig. 4), where the ratio of CT
to LE intensity showed a slight decrease from CHX through
DEE to THF, but a sharp increase in ACN. In addition, the
variation of CT emission energy of DMDMBA also clearly shows
a different profile from those of DMBA and its para-, meta- and
ortho-methyl substituted derivatives (Fig. 4), where a sharp blue-
shift was observed in THF in DMDMBA. These results reveal
the solvent-dependent competitive CT emission of DMDMBA,
namely, in the nonpolar solvents CHX and DEE the CT emission
is BA-like, whose CT occurred from the amido aniline moiety to
the benzoyl moiety,15–22 however, in polar solvents THF and ACN
the CT emission is the DMABA-like, in which the dimethylamino
group is the electron donor and the rest of molecule is the electron
acceptor.13,17 It should be also noted that the CT fluorescence
emission of DMOMBA in ACN appears at a similar wavelength
region with that of DMDMBA, however, the intensity ratio of
CT to LE is much smaller than that in the case of highly steric
DMDMBA. This is at good agreement with the enhancement
CT reaction by ortho,ortho-dimethyl substitution as observed by
Rettig et al.,25,26 reflecting the important role of steric effect played
in the CT reaction.
Fig. 2 Fluorescence spectra normalized at LE emission of DMOMBA
(a) and DMDMBA (b) in various organic solvents. CHX, cyclohexane;
DEE, diethyl ether; THF, tetrahydrofuran; and ACN, acetonitrile. The
fluorescence spectra of DMABA in ACN (dashed line) are presented in
Fig. 2b for comparison.
Fig. 4 Variations of the CT to LE emission intensity ratio (up) and CT
emission energy (bottom) of DMBAs with different substitutions at the
amido aniline moiety in CHX, cyclohexane; DEE, diethyl ether; THF,
tetrahydrofuran; and ACN, acetonitrile.
The unusual solvent dependence of ICT fluorescence of dual
emission behavior can be illustrated by the help of Scheme 2.
The DMDMBA molecule at the LE state (LE, Scheme 2) could
return to its ground state by emitting LE fluorescence hnLE from
LE, BA-like (hnCT1) and DMABA-like (hnCT2) CT fluorescence
Fig. 3 Fluorescence spectra normalized at LE emission of DMDMBA
in DEE with increasing the compositions of ACN from 0 to 18% with
interval 1%.
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fluorescence of DMDMBA was found to be different from those
of para-, meta- and ortho-methyl substituted DMBA derivatives,
and ascribed to the DMABA-like emission, where the CT reaction
occurred from the dimethylamino group to the benzanilide moiety.
The solvent-dependent CT fluorescence emission behavior of
DMDMBA was explained by consideration of, with increasing
solvent polarity, both a drastic decrease of energy barrier from LE
to DMABA-like CT state and an efficient enhancement of internal
conversion process from BA-like CT state to ground state. These
findings reveal a steric hindrance effect can play an important role
in the CT reaction that may alter the property of electron donor
and/or acceptor, but possibly also change the activation energies
of the excited state reactions. These types of examinations could be
helpful for a deeper understanding of the photophysical behavior
of DMABA and BA, as well as other compounds with competitive
CT channels, which may also contribute in the future to design of
novel balance-like ICT dual fluorescent probes.29
Scheme 2 Illustration of solvent-dependent CT dual fluorescence of
DMDMBA in nonpolar solvent CHX (solid line) and polar solvent ACN
(dashed line).
from CT states (CT1 and CT2, Scheme 2). In the nonpolar solvent
CHX, besides LE fluorescence emission, CT1 can be populated
via internal conversion (IC) process due to very low energy barrier
between the LE and CT1, but high energy barrier between the LE
and CT2 does not allow efficient population of the CT2, so that
only the BA-like CT fluorescence was observed. In the case of
polar solvent ACN, while the LE was slightly stabilized and emits
somewhat red-shifted LE fluorescence, both CT1 and CT2 were
stabilized much more effectively, and even more importantly the
energy barrier between LE and CT2 was considerably decreased
with increasing solvent polarity.28 Consequently, both the CT1
and CT2 can be populated via the IC process from LE, however,
while the strong DMABA-like CT fluorescence emission was
observed, the BA-like CT fluorescence was seriously quenched
due to the large stabilization of CT1 efficiently enhanced the IC
process between CT1 and ground state. The fluorescence quantum
yield of DMDMBA (0.0156) is smaller than that of DMABA
(0.020)13 in ACN, which is in agreement with the existence of
the additional BA-like CT deactivation channel in DMDMBA,
where an IC process from CT2 to CT1 might also be possible.
The more efficient IC process between CT1 and ground state in
DMDMBA could be readily understood from its lower energy
CT1 state, which was revealed by longer BA-like CT fluorescence
emission in DMDMBA (480 nm) than those of the para-, meta-
and ortho-methyl substituted DMBA derivatives (465 nm, 452 nm
and 460 nm, respectively) in CHX. These findings show that the
steric hindrance effect plays an important role in the CT reaction
that may alter the property of electron donor and/or acceptor, but
possibly also change the CT reaction potential.
Acknowledgements
We thank Dr C.-J. Wang for geometry calculation. This work was
supported by the National Natural Science Foundation of China
through grants No. 20175020 and The Fundamental Research
Funds for the Central Universities (DHU-2011D10511).
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