Effect of microheterogeneous media on the fluorescence and fluorescence probe properties of donor-acceptor diarylbutadienes

Article abstract of DOI:10.1039/b000586j

Diarylbutadienes, namely 1-(para-cyanophenyl)-4-phenylbuta-1E,3E-diene (1), 1-(para-methoxyphenyl)-4-phenylbuta-1E,3E-diene (2), 1-(para-cyanophenyl)-4-(para-methoxyphenyl)-buta-1E,3E-diene (3) have been prepared and investigated for their absorption and

Full text of DOI:10.1039/b000586j

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E†ect of microheterogeneous media on the Ñuorescence and
Ñuorescence probe properties of donor–acceptor diarylbutadienes
Anil K. Singh* and Sriram Kanvah
Department of Chemistry, Indian Institute of T echnology, Bombay, Powai, Mumbai-400 076,
India. E-mail: Retinal=ether.chem.iitb.ernet.in
Received (in Montpellier, France) 28th January 2000, Revised manuscript received 17th May 2000,
Accepted 17th May 2000
Published on the Web 30th June 2000
Diarylbutadienes, namely 1-(para-cyanophenyl)-4-phenylbuta-1E,3E-diene (1), 1-(para-methoxyphenyl)-4-
phenylbuta-1E,3E-diene (2), 1-(para-cyanophenyl)-4-(para-methoxyphenyl)-buta-1E,3E-diene (3) have been prepared
and investigated for their absorption and Ñuorescence properties in homogeneous media of organic solvents,
dioxaneÈwater binary mixtures and in the microheterogeneous media of sodium dodecyl sulfate, cetyl trimethyl
ammonium bromide and Triton-X-100 micelles. The Ñuorescence behaviour in various media has been correlated
in terms of empirical solvent parameters such as Dimroth E (30) and KamletÈTaft p* values, and Kirkwood
T
functions. A Linear correlation between solvent polarity and Ñuorescence properties is observed. In
ethanolÈmethanol (1 : 1, v/v) matrix at 77 K, a blue-shifted Ñuorescence maximum with enhanced Ñuorescence
quantum yields is observed for dienes 2 and 3. The singlet excited state of diene 3 is found to be highly polar in
nature. The Ñuorescence peak intensity of dienes 1 and 2 and the Ñuorescence emission wavelength maximum (j
max) of diene 3 undergo signiÐcant changes upon changing the concentration of surfactants. The Ñuorescence
f
properties of these dienes have been further used to characterize the micelles in terms of their critical micelle
concentration and micropolarity.
groups such as methoxy or cyano do not exhibit large solvent
polarity dependent changes in their Ñuorescence maximum,
thereby limiting their use as sensors and reporters of the
Introduction
Fluorescence probes as sensors and reporters of microenviron-
ments are of much current interest.1 However, only a few
microenvironment of organized assemblies.
mechanisms and sensing systems have been described. In this
We have now observed that for monosubstituted dienes
context, Ñuorescence from twisted intramolecular charge
transfer (TICT) excited states appears interesting.2 Since TICT
emission intensity and peak positions are known to be greatly
bearing either a donor (e.g. OMe) or an acceptor (e.g. CN)
group on the phenyl ring, while the changing surfactant con-
centration does not signiÐcantly a†ect the probesÏ Ñuorescence
a†ected by solvent polarity and polarizability, compounds
emission wavelength maximum (j max), signiÐcant change in
f
capable of exhibiting TICT Ñuorescence may be used as
sensors, chromogenic indicators and reporters of micro-
environments. The molecules in TICT excited states have rela-
tively large dipole moments and consequently the TICT
excited state is inÑuenced by the electronic properties of sub-
stituents, and is stabilized in polar environments.
the Ñuorescence peak intensity of the probe diene occurs. Such
concentration dependent changes in Ñuorescence intensity of
disubstituted dienes bearing both a donor and an acceptor
(e.g. OMe and CN) is not observed. However, for such substi-
tuted dienes, the Ñuorescence peak position (i.e. j max)
f
changes with changing surfactant concentration. The j max,
f
Recently, we have reported that suitably tailored donorÈ
acceptor 1,4-diarylbutadienes can also exhibit TICT
Ñuorescence3 and the TICT emission so observed for nitro-
substituted diarylbutadienes can be employed for character-
izing the microenvironment of micelles.4 These photophysical
investigations provided valuable information about the
excited state nature of a,x-diphenylpolyenes5 which have
attracted a great deal of attention in recent years because of
their relevance to linear retinylidene polyenes that are
involved in biological sensory and energy transductions.6
Further, these studies gave new directions for the development
of a,x-diphenylpolyene-based Ñuorescence probes as sensors
and reporters of microenvironments of organized assemblies.
The excited state structure and dynamics of
diphenylpolyenes are, however, not clearly known and the
sensing applications are very limited. The known probe appli-
cations of diarylbutadienes are based on the environment
polarity sensitiveness of the charge transfer Ñuorescence
maximum of the probe dienes bearing a strong electron with-
drawing group on one of their phenyl rings.4 It has, however,
been found that the dienes substituted with donorÈacceptor
in the case of disubstituted diene, and the Ñuorescence peak
intensity, in the case of monosubstituted dienes, could be uti-
lized to probe the microenvironment of micelles.
Thus, in this work, diarylbutadienes, namely 1-(para-cyano-
phenyl)-4-phenylbuta-1E,3E-diene (1), 1-(para-methoxyphenyl)
-4-phenylbuta-1E,3E-diene (2), 1-(para-cyanophenyl)-4-(para-
methoxyphenyl)buta-1E,3E-diene (3) [Fig. 1] have been pre-
pared and investigated for their absorption and Ñuorescence
properties in homogeneous media of organic solvents, 1,4-
dioxaneÈwater binary mixtures and in the micro-
heterogeneous media of sodium dodecyl sulfate (SDS), cetyl
trimethyl ammonium bromide (CTAB) and Triton-X-100
micelles. The Ñuorescence probe properties of these dienes
have been used to characterize the micelles in terms of their
critical micelle concentration (CMC) and micropolarity.
Experimental
Apparatus and general procedures
Melting points were recorded on Veego melting point appar-
atus using capillary methods and are uncorrected. IR spectra
DOI: 10.1039/b000586j
New J. Chem., 2000, 24, 639È646
639
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2000

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hyde with phosphonate esters, p-NCC H CH PO(OEt) and
6
4
2
2
p-MeOC H CH PO(OEt) in the presence of sodium
6
4
2
2
methoxide in DMF at ambient temperature. Similarly, the
reaction of para-methoxycinnamaldehyde with the phos-
phonate p-NCC H CH PO(OEt) a†orded diene 3. In a
6
4
2
2
typical procedure, 0.01 mol phosphonate, 0.015 mol sodium
methoxide and freshly distilled DMF were taken in a two-
necked round-bottomed Ñask. The mixture was stirred for
10È15 min and a DMF solution of the appropriate aldehyde
was added drop-wise under inert conditions. The stirring was
continued until most of the aldehyde had reacted as indicated
by TLC. After completion of the reaction, the reaction
mixture was quenched with brine and the organic material
was taken-up in diethyl ether. Usual work-up yielded a solid
compound that was subjected to column chromatography to
obtain the diene. Further puriÐcation of the diene was carried
out by crystallization from petroleum ether (bp 60È80 ¡C)
when yellowish needles of the dienes were obtained.
1-(para-Cyanophenyl)-4-phenylbuta-1E,3E-diene (1). This
compound, obtained in 50% yield, showed physico-chemical
data as described in literature4 [mp 158È159 ¡C (lit.,4 158È
159 ¡C)].
Fig. 1 Structure of dienes 1È3.
were measured on an Impact 400 Nicolet FTIR spectropho-
1-(para-Methoxyphenyl)-4-phenylbuta-1E,3E-diene (2). Yield
40%; mp 166È167 ¡C; UV-vis (THF): jmax, 340 nm, (e, 65 909
tometer in KBr discs. 1H NMR spectra in CDCl were
3
recorded on a 300 MHz FT NMR using SiMe as internal
4
mol~1 cm~1 l); 1H NMR (CDCl , 300 MHz): d 3.80 (3H, s,
standard. UV-vis measurements were done on Hitachi U-2000
3
methoxy
protons),
6.62
(2H,
d,
J \ 14.83
Hz,
or Shimadzu UV-260 spectrophotometers. Fluorescence mea-
surements were performed on a Spex-Ñuorolog spectro-
Ñuorimeter equipped with accessories for low-temperature
emission studies. Fluorescence measurements at low tem-
peratures were made using ethanolÈmethanol (1 : 1 v/v) glass.
PhCH2CHCH2CHC H OMe), 6.83 (1H, dd, J \ 14.83/10.43
Hz, p-MeOC H CH2CH), 6.94 (1H, dd, J \ 15.19/10.43 Hz,
6
4
6
4
p-PhCH2CHCH2), 7.21È7.35 (5H, m, phenyl protons), 6.87
(2H, dd of AB quartet, J \ 8.79/1.83 Hz, p-methoxyphenyl
protons), 7.38 (2H, dd of AB quartet, J \ 8.79/1.83 Hz, p-
methoxyphenyl protons).
Quantum yields of Ñuorescence (U ) were determined using
f
quinine sulfate as standard.7 For all the electronic spectro-
scopic studies (absorption, Ñuorescence excitation and
emission), 1.0 ] 10~5 M solutions of the dienes were used. A
Branson B-12 soniÐer (450 W, 15 ¡C, 5 min) was used for soni-
Ðcation of the micellar solutions. All solutions of dienes were
handled under protective red light conditions (10 W CEMA
General Electric night lamp, red colour). Thin layer chromato-
graphic (TLC) analyses were performed on silica gel plates
using a 10% ethyl acetateÈn-hexane (v/v) solvent system.
Column chromatography was done on silica gel using a 5%
ethyl acetateÈn-hexane (v/v) mixture as eluting solvent. All sol-
vents were dried and freshly distilled prior to their use.
1-(para-Cyanophenyl)-4-(para-methoxyphenyl)-buta-1E,3E-
diene (3). Yield 40%; mp 173È174 ¡C; UV-vis (MeCN): jmax,
359 nm, (e, 59 722 mol~1 cm~1 l); 1H NMR (CDCl , 300
3
MHz): d 6.59 (1H, d, J \ 15.38 Hz, p-MeOC H CH2CH),
6
4
6.71 (1H, d, J \ 15.38 Hz, p-NCC H CH2CH), 6.82 (1H, dd,
6
4
4
J \ 15.38/10.25 Hz, p-MeOC H CH2CH), 7.03 (1H, dd,
6
4
J \ 15.38/10.25 Hz, p-NCC H CH2CH), 6.89 (2H, dd of AB
6
quartet, J \ 8.79/1.83 Hz, p-methoxyphenyl protons), 7.40
(2H, dd of AB quartet, J \ 8.79/1.83 Hz, p-methoxyphenyl
protons). 7.48 (2H, dd of AB quartet, J \ 8.42/1.83 Hz, p-
cyanophenyl protons), 7.59 (2H, dd of AB quartet, J \ 8.42/
1.83 Hz, p-cyanophenyl protons).
Reagents and synthesis procedure
The starting materials and reagents (cyanobenzyl bromide,
benzyl bromide, triethyl phosphite, sodium methoxide etc. for
synthesis of dienes 1È3), 1,4-diphenylbutadiene (DPB), sur-
factants [cetyl trimethyl ammonium bromide (CTAB), sodium
dodecyl sulfate (SDS) and Triton-X-100 for micelle
preparation], quinine sulfate [for determination of U ], CDCl
Results and discussion
Absorption and Ñuorescence studies in homogeneous media of
organic solvents and microheterogeneous media of micelles and
dioxane–water system
f
3
and SiMe for NMR, and KBr for IR studies were from
The absorption and Ñuorescence spectral data of dienes 1È3 in
organic solvents of di†erent polarity are given in Table 1.
Representative Ñuorescence spectra in di†erent media are
shown in Fig. 2. As compared to the parent diphenylbutadiene
(DPB), the substituted dienes exhibit red-shifted absorption
jmax. The magnitude of shift varies with the substituents. In
contrast to nitro-substituted diphenylbutadienes, solvent
polarity does not signiÐcantly a†ect the absorption jmax of
these dienes. Among the three substituted dienes, the dis-
ubstituted diene 3 exhibits the maximum red shift in the
absorption jmax. The red-shift is attributed to the increased
conjugation due to the presence of an additional substituent
on the para position of the second aromatic ring.
4
Aldrich Chem. Co. U.S.A. Solvents such as acetonitrile
(MeCN), diethyl ether, dimethylformamide (DMF), dioxane,
ethanol, ethyl acetate, n-heptane, n-hexane, methanol (MeOH)
and tetrahydrofuran (THF), used in spectroscopic and other
studies, were obtained from Spectrochem Pvt. Ltd., Mumbai.
Petroleum ether (bp 60È80 ¡C) was from local suppliers.
Deionized, double distilled water (Millipore) was used for pre-
paring the micelle solutions. Chromatography grade silica gels
were obtained from E. Merck (India) Ltd., Mumbai.
The dienes 1-(para-cyanophenyl)-4-phenylbuta-1E,3E-diene
(1), 1-(para-methoxyphenyl)-4-phenylbuta-1E,3E-diene (2) and
1 - (para - cyanophenyl) - 4 - (para - methoxyphenyl) - buta - 1E,
3E diene (3) were synthesized from respective aldehydes and
phosphonates by a generalized EmmonsÈHorner procedure.8
Thus, 1 and 2 were obtained by the reaction of cinnamalde-
As compared to the homogeneous media of organic sol-
vents, the micellar environment does not cause signiÐcant
change in the absorption spectra of these dienes (Table 2).
640
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Table 1 UV-vis absorption and Ñuorescence data of dienes 1È3 in homogenous mediaa
Maximal wavelength/nm
U
StokesÏ shift/
cm~1
f
Diene
Solvent
Absorption
Emission
Excitation
(^0.001)
1
n-Heptane
Dioxane
THF
344
347
346
349
344
344
338
340
341
342
338
337
354
356
354
350
353
352
406
407
418
422
421
421
387
399
418
419
413
413
399
431
442
488
484
482
344
345
346
345
344
343
334
337
337
337
334
333
362
364
365
371
362
364
0.003
0.007
0.007
0.014
0.003
0.004
0.050
0.019
0.009
0.009
0.004
0.004
0.008
0.006
0.005
0.009
0.003
0.003
4439
4248
4978
4956
5260
5480
3833
4349
5402
5373
5372
5460
3186
4688
5624
7441
7667
7662
DMF
MeCN
MeOH
n-Heptane
Dioxane
THF
2
3
DMF
MeCN
MeOH
n-Heptane
Dioxane
THF
DMF
MeCN
MeOH
a 1,4-Diphenylbutadiene (DPB): abs j max 335 nm, em j 375È378 nm (in n-heptane, dioxane, methanol and acetonitrile); U :0.222 (n-
f
max
f
heptane), 0.052 (dioxane), 0.015 (methanol), 0.009 (acetonitrile).
Only a very moderate red shift in the absorption jmax of
dienes 1 and 3 is observed. The absorption and Ñuorescence
data for dienes 1È3 in various compositions of dioxaneÈwater
are presented in Table 3. The increasing amount of water in
dioxane does not alter the absorption maximum of these
dienes to any signiÐcant extent. Thus, in general, solvent
polarity does not signiÐcantly a†ect the ground state of these
dienes.
In contrast to a rather moderate solvent polarity e†ect on
the absorption spectra, signiÐcant solvent polarity e†ects are
seen in the Ñuorescence behavior of these substituted dienes.
In organic solvents and dioxaneÈwater systems, as the polarity
(relative permittivity, e) of the media is increased, the j max
f
undergoes a red shift. The maximum red shift is observed for
the disubstituted diene 3.
The dependence of StokesÏ shift on the solvent polarity is
correlated with solvent parameters like the Dimroth param-
eter, E (30) [a plot of E (30) of various solvents, as given in
T
T
ref. 9, vs. StokesÏ shift of dienes 1È3 in those solvents is given
in Fig. 3]9 and KamletÈTaft p* scale [a plot of p* values of
various solvents, as given in ref. 10, vs. l max of diene 1È3 in
f
those solvents as given in Fig. 4].10 It is observed that the
StokesÏ shift increases linearly with increase in solvent
polarity. The dienes show reasonable correlation with solvent
parameters E (30) and p* with the following regression data:
T
1 [slope \ 32.25, linear correlation (r) \ 0.8441, number of
data points (N) \ 11], 2 (slope \ 45.45, r \ 0.8366, N \ 11)
and 3 (slope \ 128.57, r \ 0.9139, N \ 11). The slope in
case of diene 3 is the highest compared to other dienes.
Fig. 2 Fluorescence spectra of diene 3 in (i) n-heptane, (ii) dioxane,
(iii) methanol, (iv) MeCN, (v) DMF.
Table 2 UV-vis absorption and Ñuorescence data of dienes 1È3 and DPB in microheterogeneous media of micelles
Maximal wavelength/nm
U
StokesÏ shift/
cm~1
f
Diene
DPB
Media
Absorption
Emission
Excitation
(^0.001)
CTAB
SDS
Triton-X-100
CTAB
SDS
Triton-X-100
CTAB
SDS
Triton-X-100
CTAB
SDS
334
333
334
350
350
350
343
340
343
365
361
365
381
379
381
421
421
418
417
395
399
483
484
468
331
329
331
346
346
346
337
335
338
361
358
359
0.148
0.108
0.166
0.012
0.007
0.020
0.022
0.017
0.042
0.013
0.008
0.018
3694
3645
3693
4818
4818
4647
5173
4095
4091
6693
7039
6029
1
2
3
Triton-X-100
New J. Chem., 2000, 24, 639È646
641

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Table 3 UV-vis absorption and Ñuorescence data of dienes 1È3 in dioxaneÈwater binary mixtures
1
2
3
StokesÏ
shift/
cm~1
StokesÏ
shift/
cm~1
StokesÏ
shift/
cm~1
j abs/
j max/
j ex/
j abs/
j max/
j ex/
j abs/
j max/
j ex/
f
f
f
% Water
nm
nm
nm
nm
nm
nm
nm
nm
nm
0
10
20
30
40
50
60
347
346
347
347
346
344
344
407
417
419
419
421
422
422
347/362
351/363
363
363
363
363
362
4248
4920
4952
4952
4952
5373
5429
340
338
340
340
339
339
338
399
417
417
417
418
418
418
337
337
336
336
336
335
338
4349
5604
5430
5430
5575
5575
5662
353
356
355
356
356
356
354
431
435
451
480
485
487
492
356
364
363
363
364
364
364
5126
5101
5996
7256
7471
7556
7923
Similarly, when the KamletÈTaft p* scale is plotted
StokesÏ shifts. The deviations from linearity in the case of
aprotic solvents like DMF can be accounted for in terms of
net stabilization of the diene in the excited state due to speciÐc
soluteÈsolvent interactions. These quantitative analyses of
solvent polarity parameters reveal that the solvated excited
state of the dienes is more stabilized in polar medium than the
ground state. Thus, the excited states of these dienes can be
of a polar nature. The solvatochromic shift of these dienes
were further analysed using the LippertÈMataga11 equation:
l [ l \ M[2(k [ k )2/hca3]F(D, n)N, where l [ l is the
against l max, a linear decrease is observed with increase
f
in the value of p*. The observed solvent correlations
are as follows:
1 (slope \ [8.10 ] 10~5, r \ [0.8250,
N \ 6), 2 (slope \ [1.42 ] 10~4, r \ [0.9052, N \ 5), 3
(slope \ [3.91 ] 10~5, r \ [0.9329, N \ 5) (Fig. 4). The
StokesÏ shifts of the dienes in DMF are excluded from the
calculation of linear correlation r in the plot of E (30) vs.
T
a
f
e
g
a
f
StokesÏ shift, k and k are excited and ground state dipole
e
g
moments respectively, k [ k \ *k (change in dipole
e
g
moment), h is PlancksÏ constant, c is the velocity of light, a is
the OnsagersÏ radius, and F(D, n) \ *f is the solvent polarity
determining factor. *f \ [(D [ 1)/(2D ] 1)] [ [(n2 [ 1)/(2n2
] 1)] where D is the relative permittivity and n is the
refractive index of the solvent. The Onsager radius for these
dienes was taken as 9 A which is a known value for similar
diphenybutadiene compounds namely 1-para-N,N-dimethyl-
aminophenyl)-4-(para-nitrophenyl)-buta-1E,3E-diene.12
For
dienes 1È3, the LippertÈMataga plot of StokesÏ shift vs. solvent
parameter *f is shown in Fig. 5. The change in dipole moment
(*k) thus obtained for dienes 1È3 are as follows: *k in Debye:
Fig. 3 A plot of Dimroth empirical solvent parameter9 E (30) vs.
T
StokesÏ shifts of dienes 1È3 in: (i) n-heptane, (ii) dioxane, (iii) THF, (iv)
DMF, (v) MeCN, (vi) 10% water in dioxane, (vii) 20% water in
dioxane, (viii) 30% water in dioxane, (ix) 40% water in dioxane, (x)
50% water in dioxane, (xi) methanol, (xii) 60% water in dioxane.
Fig. 5 LippertÈMataga11 plots of solvent parameter *f vs. StokesÏ
shifts of dienes 1È3 in: (i) n-heptane, (ii) dioxane, (iii) THF, (iv) DMF,
(v) MeCN, (vi) 10% water in dioxane, (vii) 20% water in dioxane, (viii)
30% water in dioxane, (ix) methanol.
Fig. 4 A plot of KamletÈTaft p* scale10 against l
f max
in (i) n-heptane, (ii) dioxane, (iii) THF, (iv) MeCN, (v) DMF.
of dienes 1È3
642
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1: 13.01, 2: 13.60, 3: 21.30. The LippertÈMataga plot clearly
demonstrates the solvent polarity sensitive Ñuorescence
behaviour of these dienes. These dipole moment values point
towards a polar nature for the singlet excited states of these
dienes. A rather large change in dipole moment upon excita-
tion of diene 3 indicates that its singlet excited state is certain-
ly polar in nature. Based on changes in dipole moments
excited state polarity has also been argued in other mol-
ecules.3,4,13,14
micelles. Thus, the presence of substituents as well as solvent
polarity causes a decrease in U in donorÈacceptor diarylbuta-
f
dienes. The decrease in Ñuorescence efficiency as the substitut-
ion takes place suggests increased non-radiative transitions in
the photoprocesses of diphenylbutadienes.
The StokesÏ shift for diene 3 in polar homogeneous media of
organic solvents such as DMF, acetonitrile and methanol is
greater than that in micellar media. Among the three micelles
studied, the smallest StokesÏ shift is observed in neutral
Triton-X-100. Thus, the ionic micelles of SDS and CTAB
a†ect the Ñuorescence spectra most. This can be due to inter-
actions between the micellar charges and the polar diene
Ñuorophore. Thus, while diene 3 shows signiÐcant micelle-
charge-dependent Ñuorescence peak shifts, dienes DPB, 1 and
In micellar media, j max of the parent DPB is slightly red-
f
shifted as compared to in organic solvents. Diene 1 shows a
Ñuorescence peak at around 418È421 nm, similar to its Ñuo-
rescence in polar solvents like methanol and acetonitrile. The
Ñuorescence spectrum of diene 3 in micelles is signiÐcantly
red-shifted as compared to that in non-polar n-heptane.
However, the Ñuorescence maxima in micelles are similar to
those in polar solvents like DME, MeCN and MeOH. As
compared to in polar solvents, the Ñuorescence maximum of 3
is blue-shifted in Triton-X-100 micelles (Fig. 6). In general, as
compared to the Ñuorescence of dienes 1È3 in non-polar n-
heptane, in micellar media the Ñuorescence of these dienes is
red-shifted; the maximum red shift is observed for diene 3. A
2 do not show such marked changes in their Ñuorescence j
max.
f
As compared to that in Ñuid solution at 298 K, the j max
f
of dienes 1 and 3 gets blue-shifted in ethanolÈmethanol glass
at 77 K (Table 4). Fig. 7 shows the Ñuorescence of dienes 2
and 3 at 77 K and at 298 K. However, the blue shift is more
comparison of the j max of dienes 1È3 in polar organic
f
solvent such as methanol with those in micelles reveals that
the dienes experience relatively polar environment in the
micelles.
The U of these dienes in organic solvents is generally very
f
low. The Ñuorescence efficiency of dienes 2 and 3 is generally
decreased in polar solvents. As compared to the unsubstituted
parent DPB, the substituted dienes show relatively small U .
f
The U of these dienes is relatively higher in Triton-X-100 as
f
compared to either SDS or CTAB. The Ñuorescence efficiency
is lowest in SDS micelles for all four dienes (DPB and 1È3).
DPB shows relatively greater Ñuorescence efficiency in all the
Fig. 6 Fluorescence spectra of dienes 3 in micelles of (a) CTAB, (b)
SDS and (c) Triton-X-100.
Fig. 7 Fluorescence spectra of dienes 2 and 3 at 298 K (a) and 77 K
(b).
Table 4 Fluorescence emission and excitation data for dienes 1È3 and DPB at 298 K and at 77 K in 1 : 1 (v/v) ethanolÈmethanol matrix
298 K
77 K
j max
f
j max
U
j max
f
jmax
U
f
f
f
Diene
Emission
Excitation
^0.001
Emission
Excitation
^0.001
DPB
375
418
416
478
335
347
333
357
0.018
0.007
0.005
0.006
379
412
417
436
334
328
333
360
ca. 1.0
0.500
0.401
0.509
1
2
3
New J. Chem., 2000, 24, 639È646
643

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observed in rigid matrices are Ðnely structured indicating the
absence of the stabilizing e†ects of solvent polarity, molecular
motions and related geometrical changes resulting in blue-
shifted emissions and increased Ñuorescence quantum yields.
All-trans-diphenylpolyenes belong to the C point group
2h
and their ground states are characterized by A symmetry
g
while the excited pp* states can have either A ~ or B ` sym-
g
u
metry.15 The A excited state is covalent in nature and hence
g
it is insensitive to solvent polarity. However, the B excited
state is ionic and it is expected to be stabilized in polar as well
u
as polarizable environments. The excited state properties of
these polyenes are governed by several factors including the
order of the two low-lying excited state levels, their electronic
energy gap and the inter-state mixing. The order of the two
lowest-lying excited states of longer all-trans-diphenylpolyenes
is relatively better understood. However, for 1,4-
diphenylbutadiene, the energy gap between the two lowest
excited states (2A and 1B ) is rather small and their actual
Fig. 8 Variation of l max of dienes 1, 2 and 3 with Kirkwood
f
function18 (e [ 1/2e ] 1) for dioxaneÈwater binary mixtures: (i)
g
u
dioxane, (ii) 10% water, (iii) 20% water, (iv) 30% water, (v) 40% water,
(vi) 50% water and (vii) 60% water in dioxane.
order is not clearly known. Given a rather small 2A È1B
g
u
splitting, small changes in molecular structure or solvent
orientation are expected to a†ect the relative ordering of the
lowest lying excited states and hence the excited state proper-
ties of these dienes.
pronounced (42 nm) in diene 3 bearing the cyano as well as
the methoxy group on the phenyl rings than in diene 1, which
The present results indicate that the lowest excited state of
these dienes is of B character. The B excited state, being
has only the cyano group on one of its phenyl rings. The U
dramatically increases for all the dienes at lower temperature
u
u
f
ionic in nature, is stabilized by the polar electronic e†ects of
the substituents and the polar solvents and hence the red
in the glassy matrix. For the substituted dienes, however, U is
f
far from unity though they are increased compared to U
f
shifts in the Ñuorescence. The magnitude and the trends in U
are not very clear, however. Highly solvent dependent singletÈ
f
obtained at 298 K. This indicates that even at low tem-
triplet mechanisms in donorÈacceptor diarylethene com-
pounds have been reported.16 Whether the presence of
donorÈacceptor groups promotes intersystem crossing leading
to the formation of triplets and hence diminished Ñuorescence
in these dienes is not presently clear. The reduced and varying
Ñuorescence efficiency can also be due to state switching. In
peratures the non-radiative transitions tend to decrease the
Ñuorescence in substituted dienes. All the Ñuorescence spectra
non-polar solvents, the Ñuorescent excited state can be of 2A
g
nature. Since the 2A ~ ] A transition is symmetry forbidden,
g
g
the resulting emission is weak.
Micropolarity and CMC of SDS, CTAB and Triton-X-100
micelles as probed by donor–acceptor diarylbutadienes
Microheterogeneous media of surfactant assemblies are
known to inÑuence the excited state properties of linear poly-
enes.17 Using the charge transfer Ñuorescence properties of the
nitro-substituted dienes, we have recently reported the deter-
mination of the relative permittivity and CMC of micelles.4
Because of the lack of strong charge transfer Ñuorescence in
monosubstituted dienes 1 and 2, we have used the changes in
the Ñuorescence peak intensity of these dienes to determine
the CMC and relative permittivity of the micelles incorpor-
ating the dienes. As reported earlier, the relative permittivity
of micellar domains incorporating the dienes, was determined
by Ðrst investigating the Ñuorescence characteristics of these
dienes in dioxaneÈwater binary mixtures.4 Diene 3 shows a
maximum increase of about 61 nm in j max as the percentage
f
of water is increased to 60%. However, for a 60% water
content dienes 1 and 2 show an increase of only about 15È19
nm (cf. Table 3).
Changing the composition of the binary solvent mixtures is
expected to bring about changes in the electrostatic solute/
solvent interactions that can be analyzed in terms of the Kirk-
wood function (e [ 1/2e ] 1)18 relating relative permittivity (e)
of solvents. The Kirkwood correlation in terms of an electro-
static model is used to show that the energy associated with
charge or dipole varies according to relative permittivity func-
tion. In order to describe such electrostatic solvent e†ects, we
plotted Ñuorescence wavenumber (l max) of dienes 1È3 vs.
Fig. 9 Variation of the Ñuorescence intensity of dienes 1 and 2 with
surfactant concentration. CTAB (L), SDS (…).
f
Kirkwood parameter (e [ 1/2e ] 1) and the results are shown
644
New J. Chem., 2000, 24, 639È646

View Article Online
in Fig. 8. In all three cases, a linear correlation is observed
structured Ñuorescence with a signiÐcant increase in intensity
in the premicellar region. The increase in Ñuorescence inten-
sity of dienes 1 and 2 corresponds to the formation of prem-
icellar aggregates due to the interaction of monomeric
surfactant with probe molecules.20 The Ñuorescence intensity
changes for monosubstituted dienes are predominant in ionic
micelles, while in the micelles of Triton-X-100, the intensity
with the largest slope in case of diene 3 (slope \ [1.1 ] 10~2,
r \ [0.9062, N \ 7) as compared to the dienes
(slope \ [3.46 ] 10~4, r \ [0.9499, N \ 6) and
1
2
(slope \ [3.8 ] 10~4, r \ [0.9830, N \ 7) indicating the
more polar nature of its excited state.
Perusing the data of Fig. 8 and comparing the Ñuorescence
of micelle-incorporated diene 3 with that of dioxaneÈwater-
incorporated diene 3, it is inferred that the diene is located in
the polar interfacial domain of the micelle especially for
changes observed do not show
a regular trend. Thus,
diphenylbutadienes bearing relatively weak donorÈacceptor
substituents can act as probes in ionic micelles.
CTAB and SDS micelles. Since the Ñuorescence j max of
f
diene 1 in micelles is similar to that observed in polar meth-
Conclusion
anol, it is further believed that these dienes are also located in
the interfacial region, while diene 2 occupies a hydrophobic
site especially in SDS and Triton-X micelles.
In conclusion it can be said that the present study has brought
out hitherto unknown aspects of the excited state of donorÈ
acceptor diarylbutadienes. The Ñuorescence of donorÈacceptor
1,4-diarylbutadienes in polar solvents is from the polar B
excited state. The electronic e†ects of the substituents and the
solvent polarity greatly inÑuence the relative ordering of the
two lowest-excited states of these dienes. It has also been
found that the dienes bearing relatively weaker donorÈ
acceptor groups do not exhibit dramatic solvent polarity
based shifts in their j max. However, such dienes exhibit
micellar charge dependent changes in their Ñuorescence inten-
sity in the micellar environment. These Ñuorescence properties
of the dienes can be utilized to investigate the microenviron-
ment of ionic micelles and related organized assemblies. In
terms of the development of charge on the Ñuorophore and
consequent polarity in the excited state, this study brings out
interesting features of the electronically excited state potential
energy surface of linear polyenes. Besides, the results can be
useful in developing newer molecular systems as environment
polarity probes and all-optical ultrafast switching mecha-
nisms.2,21
For determining the CMC values, Ñuorescence intensities of
the micelle-constituted dienes 1 and 2 were plotted against the
varying concentration of the surfactants (CTAB and SDS)
u
(Fig. 9). For diene 3, a plot of changing j max vs. varying
f
concentration of surfactant CTAB and SDS was made (Fig.
10). The CMC values calculated thus are given in Table 5. The
CMC values so obtained are in fair agreement with the CMC
values reported in the literature.19 Since the critical concentra-
tion of micelles is also dependent on the stereo-electronic
nature of the Ñuorophores, slight variation in the values of
CMC are expected. Below the CMC, dienes 1 and 2 show
f
Acknowledgements
Research grant [37/7/95-R&D-II/59] from the Board of
Research in Nuclear Sciences (BRNS), Department of Atomic
Energy, Government of India is gratefully acknowledged. The
authors are thankful to reviewers of this paper for their valu-
able suggestions.
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