Effect of coaggregate formation on the fluorescence quenching of anthracene derivatives by m-N,N-diethyl-aminophenyl carboxylates with different chain lengths as quenchers

Article abstract of DOI:10.1002/(SICI)1099-1395(199910)12:10<735::AID-POC185>3.0.CO;2-1

In the dioxane-H₂O system, electron-transfer quenching processes have been observed between the excited 9-anthrylmethyl esters of butyric acid (A-4), caprylic acid (A-8), lauric acid (A-12) and palmitic acid (A-16) as fluorescence probes, and m-N-N-diethylaminophenyl esters of butyric acid (P-4), caprylic acid (P-8) lauric acid (P-12) and palmitic acid (P-16) as quenchers. The results indicate that the hydrophobic-lipophibic interaction (HLI)-driven coaggregation of an acceptor and a donor can very effectively facilitate the electron-transfer quenching process between the excited acceptor (or donor) and the ground-state donor (or acceptor) after they become preassociated inside the coaggregate species. Furthermore, the extent of HLI-driven coaggregation (preassociation) between the acceptor aid the donor may be assessed from the slope B of the equation I₀/I = A + B[Q]. The chain-length effect and the effect of solvent aggregating power were also observed. Copyright

Full text of DOI:10.1002/(SICI)1099-1395(199910)12:10<735::AID-POC185>3.0.CO;2-1

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 12, 735–740 (1999)
Effect of coaggregate formation on the ¯uorescence
quenching of anthracene derivatives by m-N,N-diethyl-
aminophenyl carboxylates with different chain lengths as
quenchers
Ji-Liang Shi,* Zhi-Hai Qiu and Xi-Kui Jiang
Shanghai Institute of Organic Chemistry, 354 Fenglin Lu, Shanghai 200032, China
Received 16 November 1998; revised 6 April 1999; accepted 20 May 1999
ABSTRACT: In the dioxane–H O system, electron-transfer quenching processes have been observed between the
2
excited 9-anthrylmethyl esters of butyric acid (A-4), caprylic acid (A-8), lauric acid (A-12) and palmitic acid (A-16)
as fluorescence probes, and m-N-N-diethylaminophenyl esters of butyric acid (P-4), caprylic acid (P-8), lauric acid
(
P-12) and palmitic acid (P-16) as quenchers. The results indicate that the hydrophobic–lipophibic interaction (HLI)-
driven coaggregation of an acceptor and a donor can very effectively facilitate the electron-transfer quenching process
between the excited acceptor (or donor) and the ground-state donor (or acceptor) after they become preassociated
inside the coaggregate species. Furthermore, the extent of HLI-driven coaggregation (preassociation) between the
acceptor and the donor may be assessed from the slope B of the equation I /I = A ‡ B[Q]. The chain-length effect and
0
the effect of solvent aggregating power were also observed. Copyright  1999 John Wiley & Sons, Ltd.
KEYWORDS: coaggregate; anthracene derivatives; m-N,N-diethylaminophenyl carboxylates; fluorescence
quenching
INTRODUCTION
To our knowledge, these observations have not been
reported previously. The fact that the critical coaggregate
concentration (CoCAgC) value is so small may be
considered a consequence of the high aggregating
tendencies of the aggregators (Agrs). Clearly, HLI-
driven processes such as self-aggregation and excimer
formation on the one hand, and HLI-driven processes
such as co-aggregation and electron transfer on the other,
differ only in the fact that the former involves only one
Agr, whereas the latter involves two different kinds of
Agrs.
Hydrophobic–lipophilic interactions (HLI) play an im-
portant role in chemical and biochemical processes such
as conformational changes of biopolymers, the binding of
a substrate to an enzyme, the formation of living cells
1b
1
–3
from biological molecules, etc.
On the other hand,
studies on photophysical and photochemical processes in
organized assemblies have also attracted much atten-
4
–6
tion.
Since aggregates of electrically neutral organic
molecules are formed almost solely by HLI, some recent
efforts have been dedicated to studies of hydrophobic
effects on photochemical and photophysical processes.
Electron transfer is a very active topic in chemical
7–9
12
research,
such as the use of solar energy and
It has been reported that aggregation, coaggregation and
self-coiling of organic molecules affect the formation of
excimers, the enhancement of energy transfer between
photosynthetic process. It is also well known that
medium effects, such as solvent polarity, microheter-
ogeneous environment and the addition of salts, have an
important influence on the behavior of photoinduced
electron transfer between the donor and the acceptor in
solution. The present work broadens and extends
studies on hydrophobic acceleration of electron-transfer
processes facilitated by HLI-driven aggregation to the
study of the electron-transfer process between excited
9-anthrylmethyl carboxylates (A-n,n = 4, 8, 12, 16) and
m-N,N-diethylaminophenyl carboxylates (P-n,n = 4, 8,
9
,10
excited donors and acceptors
and the hydrophobic
1
1
acceleration of electron transfer processes. In the
present work, a novel finding is that at low concentrations
13
�
6
(
ꢀ10 M) *A-n can be used as an acceptor when P-n
was used as a donor, but when S-12 was used as an
acceptor, *A-12 becomes a donor in electron-transfer
quenching process driven by HLI (see structures below).
12, 16) in order to demonstrate that aggregation can also
exert a facilitating effect on the electron transfer process.
In the present work, excited 9-anthrylmethyl carboxy-
*
Correspondence to: J.-L. Shi, Shanghai Institute of Organic
Chemistry, 354 Fenglin Lu, Shanghai 200032, China.
Copyright  1999 John Wiley & Sons, Ltd.
CCC 0894–3230/99/100735–06 $17.50

7
36
J.-L. SHI, Z.-H. QIU AND X.-K. JIANG
lates with different chain lengths (A-n,n = 4, 8, 12, 16)
were used as acceptors and fluorescence probes, and m-
N,N-diethylaminophenyl carboxylates (P-n,n = 4, 8, 12,
can be facilitated by HLI-driven coaggregation. Fluores-
cence can be quenched by electron transfer or energy
transfer (or both electron and energy transfer). Quenching
were carried out by exciting A-n at 360 nm with P-n (or
S-12). The ꢀex value is larger than the P-n (or S-12)
absorption wavelength. This indicated that the energy
transfer from *A-n to P-n (or S-12) is endothermic, and
the most probable quenching mechanism is electron
transfer. In A-n–P-n systems, exciplex formation was not
observed under our experimental conditions because of
16) as donors and quenchers. HLI-facilitated electron-
transfer processes between them were investigated by
means of fluorescence spectroscopy in dioxane (DX)–
H O systems with different Φ values, where Φ is the
2
volume fraction of the organic component of an aqueous–
organic mixture.
The occurrence of electron transfer between the
excited acceptor and the ground-state donor results in
fluorescence quenching of the excited acceptor A-n* by
P-n. This is an expected result because it is well known
that intermolecular electron transfer can occur between
the lowest excited singlet state anthracene as the acceptor
12b
the polarity of the medium, as Weller
lenghi have discussed previously.
and Otto-
12c
Molecules may associate in solution for two reasons,
by van der Waals force and by collision-induced weak
charge-transfer (CT) interactions. However, both inter-
actions are weak. On the other hand, previous studies
have established that preassociation of chromophores
1
2b
and N,N-diethylaniline as the donor.
It is also well
known that the electron-transfer process facilitated by
HLI-driven aggregation is very similar to the static
quenching process that occurs with fluorophores already
associated with the quencher inside the coaggregate
15
driven by HLI can greatly facilitate excimer formation.
We speculate that enhancement of A-n* fluorescence
quenching might also be facilitated by HLI-driven
coaggregation of A-n* and P-n at very low concentra-
1
4
species. We have now experimentally demonstrated for
�
5
the first time that fluorescence quenching of A-n* by P-n
tions (ca. 10 M). Furthermore, the efficiency of
Scheme 1
Copyright  1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 735–740 (1999)

FLUORESCENCE QUENCHING OF ANTHRACENE DERIVATIVES
737
fluorescence quenching should depend on the chain
length of the substitutent groups of the acceptor probes
for C H O : C 83.03, H 8.77; found: C 82.82, H 8.84%,
27 34 2
UV–Vis absorption: ꢀmax (e) 347 nm (4600
�
1
� 1
� 1
� 1
(A-n*) or the donor quenchers (P-n*) and on the solvent
l mol cm ), 365 nm (7000 l mol cm ), 385 nm
�
1
� 1
aggregating power (SAgP) of the reaction media. All
these speculations have been found to be true.
(6400 l mol cm ) (dioxane).
9
-Anthrylmethyl ester of palmitic acid (A-16). Greenish
1
solid, m.p. 72–73°C. H NMR (CDCl ): ꢁ 0.9–1.7 (m,
3
EXPERIMENTAL
29H), 2.3 (t, 2H), 6.1 (s, 2H), 7.4–8.4 (m, 9H), anal. calcd
for C H O : C 83.36, H 9.48; found: C 83.38, H 9.64%;
31 42 2
1
Apparatus. H NMR spectra were obtained at 90 MHz on
UV–Vis absorption: ꢀmax (e) 347 nm (1200
�
1
� 1
� 1
� 1
a Varian FX-90Q spectrometer with TMS as the internal
standard. Chemical shifts are expressed in ppm (ꢁ). UV–
Vis spectra were recorded on a Perkin-Elmer 559
spectrometer.
l mol cm ) 365 nm (1600 l mol cm ) 385 nm
�
1
� 1
(1400 l mol cm ) (dioxane).
m-N,N-Diethylaminophenyl ester of butyric acid (P-4).
1
Colorless liquid. H NMR (CCl ), ꢁ 1.2 (m, 9H), 1.7 (m,
4
Reagents and substrates. All target compounds were
2H), 2.4 (t, 2H), 3.3 (q, 4H), 6.2–7.0 (m, 4H); anal. calcd
for C H NO : C 71.46, H 8.99, N 5.95; found: C 71.47,
prepared in our laboratory and identified later by
14
21
2
1
elemental analysis and H NMR. All the esters were
H 9.17, N 6.06%; UV–Vis absorption, ꢀ
(e) 260 nm
max
�
1
� 1
� 1
� 1
purified by flash column chromatography on silica gel
with light petroleum–ethyl acetate as eluent.
(31 000 l mol cm ), 304 nm (4600 l mol cm )
(dioxane).
Preparation of substrates. A-n and P-n were prepared by
the reactions shown in Scheme 1.
m-N,N-Diethylaminophenyl ester of caprylic acid (P-8).
1
Greenish liquid. H NMR (CCl ), ꢁ 1.0–1.8 (m, 19H), 2.4
4
A typical procedure for preparing the target com-
pounds is as follows. A mixture of lauric acid and excess
thionyl chloride in anhydrous benzene was stirred and
refluxed for 3–4 h and excess thionyl chloride and
benzene were removed under reduced pressure. A
benzene solution of 9-anthracenemethanol (or 3-diethyl-
aminophenol) and pyridine was added and stirred and
refluxed for another 2–3 h. The solution was washed with
(t, 2H), 3.3 (q, 4H), 6.2–6.9 (m, 4H); anal. calcd for
C H NO : C 74.18, H 10.03, N 4.81; found: C 74.25, H
18
29
2
10.30, N 4.86%; UV–Vis absorption, ꢀ
(25000 l mol cm ), 304 nm (3600 l mol cm )
(dioxane).
(e) 260 nm
max
�
1
� 1
� 1
� 1
m-N,N-Diethylaminophenyl ester of lauric acid (P-12).
1
Greenish liquid. H NMR (CCl ), ꢁ 0.9–1.8 (m, 27H), 2.4
4
saturated NaHCO and saturated NaCl solution, dried
(t, 2H), 3.3 (q, 4H), 6.3–6.9 (m, 4H); anal. calcd for
C H NO : C 76.03, H 10.73, N 4.03; found: C 75.94, H
3
with anhydrous Na SO and the solvent was removed
2
4
22 37
2
under reduced pressure. The product was purified by flash
column chromatography on silica gel with light petro-
leum–ethyl acetate as eluent. The preparation of S-12 has
11.03, N 4.08%; UV–Vis absorption; ꢀ
(41000 l mol cm ), 305 nm (7000 l mol cm )
(dioxane).
(e) 260 nm
max
�
1
� 1
� 1
� 1
1
6
been reported elsewhere.
m-N,N-Diethylaminophenyl ester of palmitic acid
(P-16). Colorless solid, m.p. 34–35°C. H NMR (CCl ),
ꢁ 0.9–1.8 (m, 35H), 2.4 (t, 2H), 3.4 (q, 4H), 6.3–7.0 (m,
1
9
-Anthrylmethyl ester of butyric acid (A-4). Greenish
4
1
liquid. H NMR (CDCl ), ꢁ 1.1 (t, 3H), 1.8 (m, 2H), 2.5
3
(t, 2H), 6.2 (s, 2H), 7.4–8.6 (m, 9H); anal. calcd for
4H); anal. calcd for C H NO : C 77.37, H 11.24, N
26
45
2
C H O : C 81.99, H 6.52; found: C 81.57, H 6.52%;
3.47; found: C 76.76, H 11.58, N 3.16%; UV–Vis
1
9 18 2
�
1
� 1
UV–Vis absorption, ꢀmax (e) 347 nm (4500
absorption, ꢀmax (e) 260 nm (50 000 l mol cm ),
�
1
� 1
� 1
� 1
� 1
� 1
l mol cm ) 365 nm (6700 l mol cm ) 385 nm
(
304 nm (7400 l mol cm ) (dioxane).
�
1
� 1
6200 l mol cm ) (dioxane).
9
-Anthrylmethyl ester of caprylic acid (A-8). Greenish
RESULTS AND DISCUSSION
1
liquid. H NMR (CDCl ), ꢁ 0.8–1.7 (m, 13H), 2.3 (t, 2H),
3
6
8
.1 (s, 2H), 7.4–8.3 (m, 9H), anal. calcd for C H O : C
Previous work has shown that HLI can facilitate the
electron-transfer process between excited 1-a-naphthyl-
2
3 26 2
2.60, H 7.84; found: C 82.74, H 7.82%; UV–Vis
�
1
� 1
absorption: ꢀmax (e) 347 nm (3200 l mol cm ),
3-oxaalkanes and 2-alkyl-3,5,6-trichloro-1,4-benzo-
�
1
� 1
14
365 nm
(4700
l mol cm ),
385 nm
(4300
quinones in DX–H O systems with different Φ values.
2
�
1
� 1
l mol cm ) (dioxane).
In order to avoid the self-quenching of the fluorescence
probe A-n and other competing photochemical processes,
e.g. photoinduced dimerization of excited anthryl group,
in our experiments on the hydrophobic acceleration of the
electron-transfer process between A-n* and P-n, the
9
-Anthrylmethyl ester of lauric acid (A-12). Greenish
1
solid, m.p. 57–58°C. H NMR (CCl ), ꢁ 0.9–1.6 (m,
2
4
1H), 2.2 (t, 2H), 5.9 (s, 2H), 7.3–8.3 (m, 9H), anal. calcd
Copyright  1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 735–740 (1999)

7
38
J.-L. SHI, Z.-H. QIU AND X.-K. JIANG
�
6
Table 1. CAgC (10 M) of A-n (n = 4, 8, 12, 16) in DX±H O
2
systems with different Φ values at 35°C
Φ
0
.20
0.25
15.0
0.30
3.63
0.35
12.7
0.40
1.67
0.45
7.09
A-16
A-12
A-8
4.89
A-4
ꢁ8.64
a
Uncertainty is less than 10%.
concentration of A-n must be kept at less than its critical
aggregate concentration (CAgC) values in the DX–H O
solvent systems used, because it is generally accepted
that below the CAgC, the A-n exists in its monomeric
2
Figure 2. Effect of gradually increasing S-12 concentration
� 6
on the ¯uorescence intensity of A-12 (1.0 Â 10 M).
f = 0.30; ꢀ = 360 nm; t = 35°C
ex
form in the DX–H O system. Therefore, the CAgCs of
2
A-n were first measured by means of fluorescence
spectroscopy in DX–H O systems with different Φ
2
1
b,d
values,
and the results are given in Table 1.
been shown that in the presence of P-n in DX–H O
2
Examination of Table 1 reveals that, as expected, the
CAgC values increase with increasing Φ values (or
solvent systems, P-n cannot be excited at 360 nm, and
therefore it could not interfere with the fluorescence
emission of A-n*. Since the excitation wavelength (360
nm) is longer than the absorption wavelengths of P-n
(260 and 304 nm) and S-12 (270 nm), the energy transfer
would be endothermic. This indicates that the possibility
of energy transfer from *A-n to P-n (or S-12) could be
excluded. Hence the most probable quenching mechan-
ism is electron transfer, in harmony with previous reports
that there is an electron-transfer process between excited
anthracene and ground-state N,N-diethylaniline in non-
decreasing SAgP) for A-n in the DX–H O system, and
2
the aggregating tendencies of A- n increase with increase
in alkyl chain length.
From our experimental results in Figs 1 and 2 it is
notable that different degrees of fluorescence quenching
of all A-n* fluorophores are observed when P-n or S-12
is added.
It is very interesting that A-12* can be used as an
acceptor when P-n is used as a donor, but when S-12 is
used as an acceptor, A-12* becomes a donor in the
electron-transfer quenching process driven by HLI. This
is consistent with the point of view that the terms electron
donor and acceptor are only relative and a single species
may behave both as a donor and as an acceptor in the
12b,d,e
polar solvents.
The fluorescence intensity of the excited fluorophore,
A-12*, decreases when the quencher P-12 is added (see
Fig. 1). In this case, A-12* was the electron acceptor and
P-12 was the electron donor, and HLI-driven preassocia-
tion precedes electron transfer between the excited
acceptor and the ground donor inside the coaggregate
species. In the case (Fig. 2) with A-n* as the donor and
S-12 as the acceptor, the fluorescence intensity of A-12*
also decreased quickly with increasing amount of S-12
1
7
study on organic electron-transfer complexes.
In the fluorescence quenching experiments, the
excitation wavelength used for A-n* is 360 nm. It has
added to the DX–H O system, and hydrophobic accel-
2
eration of electron transfer is clearly demonstrated.
However, in pure DX, there is no fluorescence quenching
�
6
between A-n* and P-n at low concentrations (ꢀ10 M),
whereas in the DX–H O system there is fluorescence
2
quenching between A-n* and P-n at the same concentra-
tion, brought about by the proximity effect of the
coaggregation under the influence of HLI. From the
above experimental results, we may conclude that under
different conditions, excited anthracene derivatives may
serve either as electron acceptors or as electron donors.
This observation has not been reported previously.
I0/I vs [Q] plots show that there is a linear relationship
[
Eqn. (1)] between I /I and [Q] when [Q] is larger than or
0
Figure 1. Effect of gradually increasing P-12 concentration
�
6
equal to CoCAgC, i.e. it is similar to the Stern–Volmer
equation, i.e. I /I = 1 ‡ k t [Q]. In Eqn. (1), B is an
on the ¯uorescence intensity of A-12 (1.0 Â 10 M).
f = 0.30; ꢀ = 360 nm; t = 35°C
ex
0
q
0
Copyright  1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 735–740 (1999)

FLUORESCENCE QUENCHING OF ANTHRACENE DERIVATIVES
739
5
� 1
Table 2. B values (10 l mol ) of the quenching of A-n with P-n in DX±H O systems with different Φ values at 35°C
2
Φ
0.20
0.25
0.30
0.35
0.40
0.45
1.04
0.50
A-16 and P-16
A-12 and P-12
A-8 and P-8
A-4 and P-4
3.51
0.470
2.04
0.0357
0.747
0.209
0.0718
0.0400
0.213
/
0.0960
a
Uncertainty is less than 10%.
1
2
� 1 � 1
Table 3. k (10 l mol s
) of the quenching of A-n with P-n in DX±H O systems with different Φ values at 35°C
q
2
Φ
0
.20
0.25
1.8
0.30
0.35
0.40
0.45
0.50
A-16 and P-16
A-12 and P-12
A-8 and P-8
A-4 and P-4
66
3.9
20
1.4
8.9
0.75
38
0.67
14
4.0
/
a
Uncertainty is less than 10% (tAn* = 5.3 ns).
empirical coefficient. Therefore, we used B = k t for the
P-12 in the *A-12/P-12 system in Φ = 0.30 DX–H O (see
q
0
2
18
evaluation of k , taking the t value as 5.3 ns for *A-n.
Tables 2 and 3), we may conclude that P-12 is better than
S-12 as a quencher in the *A-12 fluorescence quenching
process driven by HLI.
Certainly, a larger B value signifies more effective
quenching of the excited acceptor (or the donor) by the
donor (or the acceptor). The results indicate that the B
values decrease with increasing Φ value (or decreasing
SAgP) for the quenching of A-12* monomer by P-12, as
q
0
I0=I ˆ A ‡ B‰QŠ
ꢂ1†
where I is the fluorescence intensity of excited A-n in the
0
absence of the quencher, I is the fluorescence intensity of
excited A- n in the presence of the quencher, and [Q] is
the concentration of the quencher.
Furthermore, in the S-12–A-12 system, in Φ = 0.30
expected. Different k values in solvents with different Φ
value are summarized in Table 3.
q
4
DX–H O at 35°C, the B value is 5.15 Â 10 and the
2
1
2
� 1 � 1
corresponding k for S-12 is 9.7 Â 10 l mol s . By
Based on the assessed k values, which are much larger
q
q
10
� 1 � 1
comparing the aforesaid k or B value of S-12 with the k
than 10 l mol s , a diffusional control fluorescence
quenching process appears to be very unlikely. There-
q
q
1
3
� 1 � 1
5
14
value (3.8 Â 10 l mol s ) or B value (2.04 Â 10 ) of
�
6
� 6
Figure 3. I /I of A-16 (1.0 Â 10 M) vs [P-16] in the DX±H O
Figure 4. I /I of the probe (1.0 Â 10 M) vs [P-n] in the DX±
0
2
0
system with different Φ values at 35°C
H O system with Φ = 0.40 at 35°C
2
Copyright  1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 735–740 (1999)

7
40
J.-L. SHI, Z.-H. QIU AND X.-K. JIANG
fore, it might be concluded that the preassociation
quenching process induced by HLI-driven aggregation
is very similar to a static quenching process. The B values
given in Figs 3 and 4 increase with decreasing Φ values,
as expected from the fact that within a limited range of Φ
values, SAgP and Φ are almost linearly related. At
larger Φ values, the tendency for aggregation and
coaggregation of the organic aggregator will be smaller.
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1
. (a) X. K. Jiang, Acc. Chem. Res. 21, 362 (1988); (b) X. K. Jiang,
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(1980).
3. C. Tanford, Proc. Indian Acad. Sci. (Chem. Sci.) 98, 343 (1989).
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2
The I /I versus [Q] plots together with the correspond-
0
4
5
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ing B values are shown in Figs 3 and 4. They indicate that
a larger B value signifies more effective fluorescence
quenching by the quencher. Figure 4 shows that in the
quenching process between A-n* and P-n, there is a
chain-length effect of the substitutent group, and Fig. 3
shows that there is an effect of SAgP on the efficiency of
fluorescence quenching. On the other hand, on the basis
of previous findings, the lifetime of the probe monomer is
unchanged with increasing quencher concentration, i.e. it
appears to be a constant within experimental uncer-
6
. (a) S. S. Atik, C. L. Kwan and L. A. Singer, J. Am. Chem. Soc. 101,
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(
8
9
(
1
1,14
tainty.
Therefore, the lifetimes of the probes were not
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1
9
1
1. J. T. Zhang, X. L. Gui, G. Z. Ji and X. K. Jiang, Chin. Chem Lett. 4,
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consequences of stepwise processes. For the measure-
ment of CAgC or CoCAgC values based on property–
concentration plots there is usually a transition region.
Since measurements at these extremely low concentra-
tions are not very precise, the intercept value at zero
concentration may not be unity.
In conclusion, the above-mentioned results indicate
that we have successfully integrated the study of
photoinduced electron transfer processes with the study
of aggregation of organic compounds. At low concentra-
7
1
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1
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1
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�
6
tion (ꢀ10 M) in DX–H O systems the fluorescence
2
15. X. K. Jiang and J. T. Zhang, Aggregation and Self-Coiling of
Organic Molecules, pp. 21–25. Shanghai Science Press, Shanghai
quenching between A-n* and P-n (or S-12) can be
facilitated by the proximity effect of the coaggregation
under the influence of HLI. Therefore, it is of importance
to find an empirical parameter, herein designated B, for
the fluorescence quenching process which may reflect the
relative magnitudes of the tendencies of the fluorescence
quenching in order to study structural effects on the
donors and acceptors.
(
1996).
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1
1
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(
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Copyright  1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 735–740 (1999)