Intramolecular charge transfer dual fluorescence of p- dimethylaminobenzoates

Article abstract of DOI:10.1016/j.saa.2004.01.011

A series of ester, p-N,N-dimethylaminobenzoates ((CH₃) ₂NC₆H₄COOR, R=n-C_nH_2n+1, n=1,2,...,8), were synthesized and their fluorescence spectra were recorded. Intramolecular charge transfer (CT) dual fluorescence was found in common organic solvent or in water with cetyltrimethylammonium bromide (CTAB) micelle. The fluorescence intensity ratio of CT band to normal band (I _CT/I_LE) was decreased dramatically with the increasing of carbon number of R group when the carbon number of R group were 1-3, but the dependence was not obvious when the carbon number of R group were 4-8. Oppositely, the I_CT/I_LE was increased with increasing of carbon number of R group in water with CTAB micelle.

Full text of DOI:10.1016/j.saa.2004.01.011

Spectrochimica Acta Part A 60 (2004) 2729–2732
Intramolecular charge transfer dual fluorescence of
p-dimethylaminobenzoates
Cai-Hong Zhanga, Zhao-Bin Chena,∗, Yun-Bao Jiangb
a
Department of Chemistry, Shanxi University, Taiyuan 030006, PR China
b
Department of Chemistry and the MOE Key Laboratory of Analytical Sciences, Xiamen University, Xiamen 361005, PR China
Received 2 November 2003; accepted 9 January 2004
Abstract
A series of ester, p-N,N-dimethylaminobenzoates ((CH3)2NC6H4COOR, R = n-CnH2n+1, n = 1, 2, . . . , 8), were synthesized and their
fluorescence spectra were recorded. Intramolecular charge transfer (CT) dual fluorescence was found in common organic solvent or in water
with cetyltrimethylammonium bromide (CTAB) micelle. The fluorescence intensity ratio of CT band to normal band (ICT/ILE) was decreased
dramatically with the increasing of carbon number of R group when the carbon number of R group were 1–3, but the dependence was not
obvious when the carbon number of R group were 4–8. Oppositely, the ICT/ILE was increased with increasing of carbon number of R group
in water with CTAB micelle.
© 2004 Elsevier B.V. All rights reserved.
Keywords: p-N,N-Dimethylaminobenzoate; Dual fluorescence; Organic solvent; CTAB micelle
1. Introduction
electro-drawing ability of electron-acceptor group. More-
over, the relative positions and intensities of the dual emis-
sion were dependent on the solvent polarity and viscosity
[9–11].
Recent years, the study of the intramolecular charge
transfer (CT) dual fluorescence has been an active area
in theoretical and practical research [1]. The investiga-
tions of the charge transfer process and the properties of
CT dual fluorescence have attracted much attention for
their potential applications in fluorescence probe, et al. It
is well know that p-(N,N-dimethylamino)benzonitrile and
p-(N,N-dimethylamino)benzoic acid are typical molecules
with intramolecular charge transfer dual fluorescence, which
containing both electron-donor and electron-accepter group
[2–4]. Many new molecules which contain N,N-dialkyl
group and different electron-withdrawing group (–CF3,
–CHO, –CONH2 and –COOH [5–8]) have been synthe-
sized and examined. The research result showed the similar
profile of dual fluorescence can be observed in molecules
with quite different electron-acceptor substituents, and
the position of the CT emission band were affected by
electron-acceptor strength. The red-shifts of CT emis-
sion band were observed along with the increasing of
In the paper, a series of ester, p-N,N-dimethylami-
nobenzoates ((CH3)2NC6H4COOR, R = n-CnH2n+1, n =
1, 2, . . . , 8) (DMABE), were synthesized. As comparing
with acid, (CH3)2NC4H6COOH, the esters can avoid acidic
dissociation and specific hydrogen bonding between the
electron acceptor and solvent [12,13]. The properties of
dual fluorescence of DMABE in organic solvents and in
water with cetyltrimethylammonium bromide (CTAB) mi-
celle were examined. The research results were helpful to
understand the CT photophysical processes.
2. Experimental
p-N,N-Dimethylaminobenzoates ((CH3)2NC6H4COOR,
R = n-CnH2n+1, n = 1, 2, . . . , 8), were synthesized by
reaction of p-N,N-dimethylaminobenzoic acid with corre-
sponding alkyl alcohol (Scheme 1). The synthesis of the
esters, p-N,N-dimethylaminobenzoates (n = 1, 2, 3), were
carried out with refluxing in corresponding alkyl alcohol.
The synthesis of the other esters (n = 4, 5, 6, 7, 8) was
∗
Corresponding author. Tel.: +86-351-7011-492;
fax: +86-351-7011-688.
E-mail address: zchen@sxu.edu.cn (Z.-B. Chen).
1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2004.01.011

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C.-H. Zhang et al. / Spectrochimica Acta Part A 60 (2004) 2729–2732
(CH3)2NC6H4COOC4H9
H3C
H3C
1500
4
COOR
N
1.CH
2.DEE
3.AAE
4.CH2Cl2
5.ACN
6.EtOH
3
1000
2.C2H5
1. CH3
R=
1
2
4. C4H9
3. C3H7
5. C5H11
7. C7H15
5
500
6. C6H13
8. C8H17
6
0
300
Scheme 1. Chemical structures of p-dimethylaminobenzoates.
350
400
450
500
550
Wavelength,nm
refluxed in the benzene solvent. The products were charac-
terized by IR data and NMR data.
Organic solvents were purified before use and were
checked to have no fluorescent impurity at the used exci-
tation wavelength. Cetyltrimethylammonium bromide (AR,
Shanghai) was used as received.
Corrected fluorescence spectra and the absorption spectra
were recorded on Hitachi F-4500 fluorescence spectropho-
tometer and Beckman DU-7400 absorption spectrophotome-
ter, respectively. Quantum yields of the aerated sample so-
lutions were measured using quinine sulfate as the standard
(0.546 in 0.5 mol l−1 H2SO4 [14]). IR data were acquired
from Nicolet Avatar FT-IR 360 spectrometer. AM1 calcula-
tions were carried out using GAUSSIAN 94 software [15].
Fig. 1. Fluorescence spectra of butyl p-dimethylaminobanzoate in solvents
of varied polarity. Solvents are cyclohexane (CH, 1), diethyl ether (DEE,
2), ethyl acetate (AAE, 3), dichloromethane (CH2Cl2, 4), acetonitrile
(ACN, 5) and ethanol (EtoH, 6), respectively.
peared any change (Table 1). The similar phenomenon was
found in other solvents.
3.2. The fluorescence spectra of the esters in organic
solvents
The fluorescence spectra of the esters were recorded
in several solvents with different polarity. The excitation
wavelength was located at 300 nm and the concentration
of these esters was lower about 1 × 10−5 mol l−1. Fig. 1,
as an example, showed the emission spectra of butyl
p-N,N-dimethylaminobenzoate in different organic solvents.
It was found that the esters emitted mainly short-wavelength
fluorescence with a tail extending to beyond 450 nm in
non-polar solvent, cyclohexane (CH). The dual fluorescence
was revealed in polar solvents and could be readily assigned
to the locally excited (LE) state and the charge transfer state,
respectively. In agreement with some reported observations
[3], the LE emission of at around 345 nm showed very weak
dependence on solvent polarity, whereas the CT emission
showed red-shifts with the increasing of solvent polarity.
In addition, the fluorescence spectra showed apparently
variation with different R group. Fig. 2 displayed the fluores-
cence spectra of different esters in CH2Cl2. It was observed
that the CT emission of some esters (R = –CH3, –CH2CH3,
–CH2CH2CH3) in CH2Cl2 were gradually blue-shifted and
3. Results and discussion
3.1. Some characterizations and absorption spectra of the
eaters
The series of esters were characterized by IR and the IR
C O
data of carbonyl (
) in different esters didn’t appear ob-
vious change (Table 1). The dipolar moments of ground-state
of the esters were calculated and about 3.88 0.2 D (Table 1).
That suggested the ground-state structures of these esters
were similar, thereby ensuring the comparability of the elec-
tron acceptors in these esters.
The absorption of these esters in common organic solvents
were measured. It was found that the maximum absorption
wavelengths of the esters in acetonitrile (ACN) hardly ap-
Table 1
The IR stretching frequencies of the carbonyl double bonds, the absorption and fluorescence spectral parameters of p-dimethylaminobenzoates in ACN
and the ground-states dipole moments
R
νc= 0 (IR, cm−1
)
λabs (max, nm)
λCT/λLE (nm)
ICT/ILE
Total quantum yield
µg (D)
–CH3
–C2H5
–C3H7
–C4H9
–C5H11
–C6
1701.2
1699.6
1702.3
1703.0
1703.0
1703.0
1701.1
1704.0
309
308
308
309
309
308
308
308
500.8/349.8
497.6/347.2
496.8/344.6
497.6/343.8
495/343.2
497.6/343.8
500.8/345.8
498.2/343.2
10.21
5.36
0.35
0.71
0.68
0.85
1.02
0.97
0.02897
0.01673
0.03819
0.03502
0.04383
0.03541
0.04470
0.05114
3.99
3.90
3.88
3.88
3.87
3.88
3.84
3.86
–C7
–C8

C.-H. Zhang et al. / Spectrochimica Acta Part A 60 (2004) 2729–2732
2731
CTAB C: 2.0*10-3
600
500
400
300
200
8
(CH3)2NC4H6COOR
R= 1.CH3 2.C2H5
3.C3H7 4.C4H9
5.C5H11 6.C6H13
7.C7H15 8.C8H17
7
6
5
1
100
4
2.3
1
0
320
360
400
440
480
520
560
Fig. 2. Fluorescence spectra of p-dimethylamonobenzoates in CH2Cl2.
Wavelength,nm
the ratio of fluorescence intensity of CT emission to LE
emission (ICT/ILE) were sharply decreased, but the CT emis-
sion of other esters (R = –C4H9 to R = –C8H17), showed
little change on either fluorescence wavelength or ratio of
fluorescence intensity (ICT/ILE). Similar phenomena were
observed in other polar solvents and were shown in Fig. 3.
Fig. 4. Fluorescence spectra of p-N,N-dimethylaminobenzoates in an aque-
ous solvent in the presence of CTAB (2.0 × 10−3 M).
with the interactions between the esters and the micelle. The
interactions may be hydrophobic interaction and electron
interaction between –COOR group and micelle [17].
3.3. Fluorescence spectra of the esters in water with CTAB
(2.0 × 103 M) micelle
4. Conclusions
In order to understand profoundly the CT emission
responding to the electron-acceptor strength or the envi-
ronmental effects, we investigated their fluorescent spectra
in water with cetyltrimethylammonium bromide micelle,
after the critical micelle concentration (CMC) [16]. Fig. 4
showed the fluorescence spectra of the esters in water with
CTAB (2.0 × 10−3 M). It can be founded that the local
emission (LE) of methyl p-N,N-dimethylaminobenzoate
((CH3)2NC6H4COOCH3) was more red-shifted than the
other esters. The result could be attributed to aggregates of
methyl ester in water. Other esters which contain more large
R group could easily embed into CTAB micelle that destroy
the aggregated state of esters in water [17]. So the intensity
ratio of CT emission to LE emission was gradually increased
with the increasing of chain length of R group. The results
demonstrated the increasing of stabilization of the CT state
A
series of esters, p-N,N-dimethylaminobenzoates
((CH3)2NC6H4COOR, R = CnH2n+1, n = 1, 2, . . . , 8),
were prepared. The IR data and the ground-states dipole
moments showed that the ground-state structures of these
esters were similar. The CT dual fluorescence were observed
in different medium. In organic solvent, the position of
CT emission of some esters (n = 1, 2, 3) was blue-shifted
and the ICT/ILE were decreased alone with the increasing
of the chain length of R group, but the CT emission of
other esters (n = 4, 5, 6, 7, 8) were very similar. In water
with cetyltrimethylammonium bromide (CTAB) micelle,
the ICT/ILE was increased with the increasing of the length
of R group.
Acknowledgements
The present work was supported by the National Science
Foundation of China (No. 30000112); Laboratory of Organic
Solid, Chinese Academy of Sciences and the Returned Stu-
dent Science Foundation of Shanxi Province of China.
10
8
DEE
AAE
CH2Cl2
ACN
EtOH
CTAB
6
References
4
[1] E. Lippert, W. Luder, H. Boos, in: A. Mangini (Ed.), Advances in
Molecular Spectroscopy. Pergamon, Oxford, 1962, p. 443.
[2] Z.R. Grabowski, K. Rotkiewicz, A. Siemiarczuk, D.J. Cowley, W.
Baumann, Nouv. J. Chim. 3 (1979) 443.
2
0
0
1
2
3
4
6
7
8
5
9
Number C in R((CH ) NC6H4COOR)
[3] W. Rettig, Angew. Chem. Int. Ed. Engl. 25 (1986) 971.
[4] W. Rettig, in: Proceedings of the 18th International Conference on
Photochemistry, Warsaw, 1997, p. I4.1.
32
Fig. 3. Plots of ICT/ILE vs. number C in R ((CH3)2NC6H4COOR).

2732
C.-H. Zhang et al. / Spectrochimica Acta Part A 60 (2004) 2729–2732
[5] W. Rettig, B. Zietz, Chem. Phys. Lett. 317 (2000) 187.
[6] Z.R. Grabowski, Book of Abstracts, in: The XXth International
Conference on Photochemistry, Moscow, 2001, p. IL5.
[7] W. Rettig, G. Wermuth, E. Lippert, Ber. Bunsenges. Phys. Chem.
83 (1979) 692.
[8] M. Niebuer, M. Grobys, I. Rücket, K.A. Zachariasse, Book of Ab-
stracts, in: Proceedings of the 17th IUPAC Symposium on Photo-
chemistry, Barcelona, 1998, p. PB-153.
[11] J.M. Hicks, M.T. Vandersall, E.V. Sitzmann, K.B. Einsenthal, Chem.
Phys. Lett. 135 (1987) 413.
[12] C. Cazeau-Dubroca, S.A. Lyazidi, P. Cambou, A. Peirigua, Ph.
Cazeau, M. Pesquer, J. Phys. Chem. 93 (1989) 2347.
[13] A. Levy, D. Avnir, M. Ottolenghi, Chem. Phys. Lett. 121 (1995) 233.
[14] J.N. Dmas, G.A. Crobys, J. Phys. Chem. 75 (1971) 991.
[15] M.J. Frisch, et al., GAUSSIAN 94, Gaussian Inc., Pittsburgh, PA,
1995.
[9] Y.H. Kim, D.W. Cho, M. Yoon, D.J. Kim, Phys. Chem. 100 (1996)
15670.
[16] M.D.G. Nascimento, M.A. Lezcano, F. Nome, J. Phys. Chem. 96
(1992) 5537.
[10] J.M. Hicks, M.T. Vandersall, Z. Babarogic, K.B. Einsenthal, Chem.
Phys. Lett. 116 (1985) 18.
[17] Y.B. Jiang, J.G. Xu, X.Z. Huang, Chem. J. Chin. Univ. (Chinese)
15 (1994) 552.