Transcript 36x60 Vertical Template

Solvent effects on the solvatochromism of 7-aminocoumarins in
neat and binary solvent mixtures: Correlation between the
electronic transition energies and the solvent polarity parameters
Jin-Young Choi, T. J. Kang
Department of Chemistry, Daegu University
Abstract
Gyoung san, 712-714 Korea
The change in the electronic absorption and emission energies of 7-aminocoumarin derivatives in binary solvent mixtures has been studied.
The electronic transition energy along with the Stokes’ shift is correlated with the orientation polarizability of the solvent as well as the
empirical solvent polarity parameters ET(30). It is observed that the emission peak shift traces polarity change very nicely in the binary solvent
mixtures. The emission transition is more strongly depend on the solvent polarity than the absorption. From the dependence of the
Sokes’shift of 7-aminocoumarins with the solvent polarity parameters and the ground state dipole moment obtained by the semi-empirical
AM1 calculations, the excited state dipole moment was estimated. The fluorescence lifetime change of 7-aminocoumarins in binary solvent
mixtures was measured and the results are explained in terms of molecular conformation and solvent polarity. The study indicates the
empirical solvent polarity ET(30) is a good measure of microscopic solvent polarity and it probes in general the non-specific solvent
interactions.
Introduction
Theorical background
Structure of 7-aminocoumarin dyes Solvent Polarity Parameters
Solvation is important for understanding
the solvent effects on chemical and
biochemical processes.
(C2H5)2N
N
Biological systems in which various
physiological
processes
such
as
transportation, signaling, metabolism are
controlled by solvation. Changes in local
polarity by solute-solvent interactions in
biological
systems
are
related
to
malfunction or disease.
O
O
① Δf(ε,n) : Orientation polarizability
O
 1
n2 1
f ( , n) 

2  1 2n 2  1
O
Coumarin 102 CH3
Solvatochromism
and
the
estimation of the dipole moment
① Lippert, Mataga
2(e   g ) 2
 a  f 
f ( , n)  C
② ET(30) : Empirical solvent polarity
4 0 hca03
●
The ET(30) measures solvent
polarity with the charge transfer ② Bakhshiev, Kawski
absorption maxima of propidium
 a  f  m1 f1 ( , n)  C
phenol betaine known as Reichardt
dye .
 a  f  m2[ f1 ( , n)  2 f 2 (n)]  C
ET (30)  h  c  N A
③ Ravi et al
ET (30) solvent  ET (30)TMS
N
ET 
 μ 2 aB 3  N
ET (30)Water  ET (30)TMS
 a  f  11308(
) ( )  ET  C
③ Other empirical polarity parameters

μ
a 
B

● Z value : Kosower ● Y-scale and π*
Coumarin 481 CF3
Effect of general solvent polarity
A variety of solvent polarity parameters
proposed. But, there is no such thing as a
single polarity parameter in explaining a
multitude of solvent effects.
Results and discussion
Correlation between ETN and Δf(ε,n)
Correlation of Stokes’shift with Δf(ε,n) and ETN
6.0
6.0
0.2
0.1
0.2
0.3
-1
-1
3
3.0
2.0
1.0
0.1
0.2
Δf(ε,n)
0.0
0.0
0.3
0.1
0.2
0.3
ET
1.4
0.0
400
450
500
550
0.8
600
The shape of the
cavity in
7-amonocoumarin
dyes is not spherical
but is very much
elliptical. Thus
whenever the cavity
radius is required for
calculations, it should
be taken very carefully.
4.0
-1
4.0
3
They show batochromic
shift as solvent polarity
increases.
0.1
0.7
6.0
2.0
2.0
3
4
0.3
0.2
0.6
1.6
6.0
6.0
-1
0.4
0.5
N
2
va- vf (×10 cm )
0.5
0.6
2
f1+3(n -1)/(n +2)
0.8
1.0
1.2
-1
0.6
0.4
va- vf (×10 cm )
0.7
4
fluorescence Spectra
of coumarin 102 were
measured in dioxane/
water binary solvent
system.
0.971
0.959
0.934
0.904
0.826
0.703
0.145
0.195
0.000
0.8
0.4
The plot of the Stokes’
shift of coumarin 102
in benzene-methanol
binary solvent as a
function of ETN is
compare to the plot of
the Stokes’shift as a
function of the Δf(ε,n).
Estimation of the excited dipole moment
(a) Absorption and (b)
Water mole
fraction
0.9
350
R=0.95347
(b) Red shift
1.0
Normalized absorption and
fluorescence spectra intensity
2.0
0.0
0.0
Solvatochromic shift of 7-aminocoumarin dye
(a) Red shift
3.0
1.0
Δf(ε,n)
1.1
R=0.90113
4.0
va+ vf (×10 cm )
0.0
0.0
4.0
3
0.4
5.0
5.0
va- vf (×10 cm )
ET
N
0.6
ET
was measured for 14
aprotic solvents and 5 protic
solvents, and plotted versus
Δf(ε,n). It is noted that there is
a linear correlation between
ETN and Δf(ε,n) for aprotic
solvents, but large deviation
is observed for protic solvents.
*
*
*
**
Fig(1)(R=0.95347)seems
to give better correlation
Than Fig(2)(R=0.9011).
(2)
(1)
N
va- vf (×10 cm )
0.8
4.0
2.0
Wavelength(nm)
Comparison of emission peak shifts with the change of ETN
1.0
0.0
0.0
0.2
0.4
22
-1
vf (×10 cm )
Compounds
0.4
f(ε,n)
20
19
0.2
ETN
(d)
0.0
0.0
1.0
0.2
0.4
0.6
0.8
18
0.0
1.0
0.2
0.4
0.6
0.8
Intensity(Counts)
3
21
20
19
0.2
(e)
0.0
0.0
1.0
0.2
0.4
0.6
0.8
18
0.0
1.0
μg **
(D)
μe/μg
4802
6.38
2.05
C481
1924
5169
6.24
2.19
μe
(D)
*Calculated
from
the molecular
13.10 volume assuming a
13.64 spherical shape
C102
2249
3.98
2.06
8.44
C481
2552
4.00
2.22
8.46
**Semi-empirical
AM1 Calculation
0.2
0.4
0.6
0.8
6
0.939
0.917
0.898
0.869
0.815
0.688
0.525
0.356
0.000
laser pulse 400nm
Methanol
mole fraction
1000
Increasing polarity
100
1.0
0
(c)
0.8
Solvent polarity and fluorescence lifetime change
-1
vf (×10 cm )
N
0.4
Δμ
(D)
1656
22
0.6
a*
(Cavity Radius, Å)
C102
10000
0.8
ET
m1
m2
m0
-1
-1
(cm ) (cm ) (cm-1)
0.6
N
1.0
23
(b)
0.4
21
3
ET
N
0.6
0.2
ET
Fluorescence Lifetime(ns)
0.8
0.0
0.0
0.0
1.0
0.8
f1
23
(a)
0.6
5
10
15
20
Time(ns)
23
0.8
25
30
35
C102
C481
5
Plotting fluorescence
lifetime against solvent
polarity indicates
characteristic
curvilinear change for
different coumarin
fluorophores.
4
3
2
1
0
0.0
0.2
0.4
0.6
ET
0.8
1.0
N
vf (×10 cm )
22
Conclusions
21
3
ET
N
-1
0.6
0.4
20
0.2
19
0.0
0.0
18
0.0
1.
(f)
0.2
0.4
0.6
Xmore polar solvent
0.8
1.0
0.2
0.4
0.6
Xmore polar solvent
0.8
1.0
2.
Fig(a)~(c), The empirical solvent polarity parameter plotted as a function of X , the
mole fraction of the more polar component of binary solvent mixtures.(d)-(f)the 3.
wavenumbers of the emission peaks for coumarin 102 dissolved in three binary
solvent mixtures plotted as a function of mole fraction of more polar solvent. The 4.
microscopic solvent polarity is probed very nicely by an empirical polarity
measure of ETN. (a) and (d) : benzene/acetonitrile mixture, (b) and (e) :
benzene/methanol mixture, (c) and (f) : dioxane/water
The solvent effect on the solvatochromism is correlated to the empirical polarity parameter ETN
better than the bulk solvent parameter function. The ETN value measures microscopic solvent polarity
and it seems to probe more or less non-specific solvent interactions.
The dipole moment 7-aminocoumarins increases by almost twice upon excitation. The cavity shape
is assumed to be very much elliptical and the cavity radius is predicted to be much larger than the
generally considered value for these 7-aminocoumarins.
Preferential solvation is taking place in binary solvent mixture since the change of peak shift seems
to reflect the change of ETN very closely.
The lifetime of coumarin 102 gradually increases with increasing solvent polarity, but the
fluorescence lifetime of coumarin 481 rapidly decreases as the solvent polarity increases. This is
attributed to the nonradiative decay process which involves the formation of twisted intramolecular
charge transfer state.