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withdrawing groups (–CN, –CO2CH3, –CHO, –
COCH3, –CONH2 [7,8,10], –CF3 [11] and –SO3Na
[12]) para-substituted N,N-dimethylanilines, the
variations in the acceptors could also change the
excited-state reaction coordinates, for instance, in
the cases with cyano and carboxylate para-substi-
tuted N,N-dimethylanilines [2]. In the series of
p-dimethylaminobenzamides [9] a steric effect was
also introduced along with the electron polar effect
when the amido moiety was changed from –
CONH2 through –CONðCH3Þ to –CONðC2H5Þ .
that CT occurred with these esters. In this Letter
we will report the electron-acceptor dependence in
the CT dual fluorescence and its implications to
the structural relaxation upon CT reaction and
the CT dual fluorescence chemosensor designing.
2. Experimental
Substituted-phenyl p-dimethylaminobenzoates
(Scheme 1) were synthesized by reaction of
p-dimethylaminobenzoic acid with substituted
phenols in the presence of POCl3. The products
were repeatedly recrystallized and characterized by
IR and NMR data. Organic solvents were purified
before use and were checked to have no fluorescent
impurity at the used excitation wavelength.
Corrected fluorescence spectra and the absorp-
tion spectra were recorded on Hitachi F-4500
fluorescence spectrophotometer and Beckman
DU-7400 absorption spectrophotometer, respec-
tively. Quantum yields of the aerated sample
solutions were measured using quinine sulfate as
the standard (0.546 in 0.5 mol LÀ1 H2SO4 [15]). IR
data of the KBr plated samples were acquired
from Nicolet Avatar FT-IR 360 spectrometer and
NMR data were taken on Varian Unityþ 500 MHz
spectrometer (CDCl3, TMS). AM1 calculations
were carried out using GAUSSIAN 94 software
[16].
2
2
In spite of these complexities the examination on
the effect of the electron acceptor variations has
indeed led to interesting observations that would
be of great importance to the understanding of the
CT photophysics [9]. We therefore attempted to
construct a series of dual fluorescent N,N-dime-
thylaniline derivatives with comparable electron
acceptors.
It has been shown that the –NH– bridge could
efficiently transmit the substituent effect [13,14].
Our first attempt was hence made to substituted-
phenyl p-dimethylaminobenzoates (Scheme 1),
hoping to achieve the variations of the electron-
accepting ability of the acceptor moiety by sub-
stitution at the ester phenyl ring while the ester
identity of the acceptor moiety remains un-
changed. In these esters the phenoxyl –O– bridge
was employed to transmit the substituent effect,
since the –O– bridge was expected to be similar to
the –NH– bridge while avoid the complicated
n–pà transition in the amide derivatives [9]. In
order to avoid the possible steric effect due to the
ortho-substitution only the para- and meta-sub-
stitutions were tried. The AM1 calculations
pointed out that no appreciable steric effect was
introduced in our series of substituted-phenyl
p-dimethylaminobenzoates (Scheme 1). Fluores-
cence spectra recorded in polar solvents indicated
3. Results and discussion
We synthesized a series of substituted-phenyl
p-dimethylaminobenzoates (Scheme 1) in which
the electron acceptors were varied by introducing
substituent at the para- or meta-position of the
ester phenyl ring in order to avoid the steric effect
while electronic polar effect was incorporated. We
here called the ester phenyl ring the mediator that
communicates via the phenoxyl –O– bridge the
electronic polar effect of the substituent with
the dimethylaminobenzoyl moiety of the molecule.
We expected that by designing this series of mol-
ecules we could generate a series of the CT dual
fluorescent esters with the same electron donor
while varied but comparable electron acceptors.
Scheme 1. Molecular structure of substituted-phenyl p-dim-
ethylaminobenzoates.