1
Photoinduced Electron Transfer between Various Coumarin analogues and N, N-1
dimethylaniline inside Niosome, A Nonionic Innocuous Polyethylene glycol-based 2
Surfactant Assembly 3
Chiranjib Ghatak, Vishal Govind Rao, Sarthak Mandal, Nilmoni Sarkar.* 4
*Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, WB, 5
India. 6
E-mail: nilmoni@ chem.iitkgp.ernet.in 7
Fax: 91-3222-255303 8
Abstract 9
Photoinduced electron transfer (ET) reactions between coumarin dyes and N, N-10
dimethylaniline have been investigated inside niosome, a nonionic innocuous 11
Polyethylene glycol (PEG)-based surfactant assemblies using steady state and time-12
resolved fluorescence measurements. The location of coumarin dyes has been 13
reported inside bilayer headgroup region of niosome and it was verified by 14
determination of high distribution coefficient of all the dyes inside niosome compared 15
to bulk water. Fluorescence anisotropy parameters of the dyes inside niosome are also 16
in good correlation with the above inference about their location. Bimolecular 17
diffusion guided rates inside niosome was determined by comparing the 18
microviscosities inside niosome and in acetonitrile and butanol solutions and it was 19
found that diffusion of donor and acceptor is much slower than the ET rates implying 20
insignificant role of reactant diffusion in ET reaction inside niosome. We have 21
obtained Marcus inversion region in our restricted media, which shows maxima at 22
lower exergonicity. Such behavior has been demonstrated by the presence of 23
nonequilibrium solvent excited state using two dimensional ET (2DET) theory. 24
Unusually high quenching rates of two coumarins C-152 and C-152A inside niosome 25
were explained by the presence of stable non-fluorescent twisted intramolecular 26
charge transfer (TICT) state along with emissive intramolecular charge transfer (ICT) 27
state. Moreover, intermolecular hydrogen bonding between carbonyl oxygen of these 28
two dyes with water in their non-emissive and emissive charge transfer states also play 29
a key role for their dynamical exchange to each other.1 30
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1. Introduction 31
Electron transfer, or the act of moving an electron from one place to another, is 32
amongst the simplest of chemical processes, yet certainly one of the most critical. The 33
unique simplicity of ET reactions has fostered the development of a powerful 34
theoretical formalism that describes the rates of these processes in terms of a small 35
number of parameters.1 Both theoretical and experimental investigations have been 36
carried out on the dynamical aspect of the photoinduced electron transfer (PET)3-37
10.These investigations, so far have been carried out are either in neat solvent where 38
the solvent acts as a donor or under diffusive condition where the solvent is non-39
interacting and reactants have to diffuse before ET takes place.10,12-14
While ET 40
processes in the homogeneous media are well known, works in organized media such 41
as micelles, reverse micelles, cyclodextrin etc. have also been performed during the 42
last few years15-19
. It would be interesting to study ET in organized media because 43
these systems bear resemblance to many biological and chemical systems in nature. 44
The time dependence of electron transfer depends on the electronic properties of the 45
donors and acceptors, as well as the structure and morphology of the local 46
environment.20-23
Typically, intermolecular electron transfer occurs on a distance scale 47
of a few Angstroms, making the process sensitive to the details of the local 48
environment.24, 25
. Recent studies on excited-state intermolecular hydrogen dynamics 49
(ESIHBD) highlighted another aspect on intermolecular (PET) and it was reported that 50
PET processes become faster in presence of strong Hydrogen bond donating solvents 51
compared to non Hydrogen bond donating solvents 1,26
. It was experimentally as well 52
as theoretically proved first time by Han et al 26d
that intermolecular hydrogen bonding 53
between solute and solvent facilitates the electron transfer processes especially when 54
ET reactions are much faster than solvation dynamics. Similar information was also 55
reported by Carlos that ET rates are higher in protic solvents (methanol, ethanol) 56
compared to aprotic solvents (acetonitrile, propionitrile).27
57
In this context we have taken ―niosome‖, a nonionic surfactant vesicle as our 58
organized assemblies to study the popular ET reaction between Coumarin derivatives 59
and N, N-dimethylaniline (DMA). Niosomes are now widely studied as an alternative 60
to liposomes, which exhibit certain disadvantages such as the fact that they are 61
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expensive, their ingredients like phospholipids are chemically unstable because of 62
their predisposition to oxidative degradation, they require special storage and handling 63
and purity of natural phospholipids is variable. Niosomes represent a promising drug 64
delivery module. They present a structure similar to liposome and hence they can 65
represent alternative vesicular systems with respect to liposomes, due to the ability to 66
encapsulate different type of drugs within their multienvironmental structure. 67
Niosomes have unilamellar as well as multilamellar vesicular structure according to 68
their preparation procedure. For this work, Tween80 with poly (ethylene glycol) 69
(PEG6000) was selected to prepare highly stable niosomes28
. PEGs are simply 70
oligomer or polymer of ethylene oxide. Tween80 is a pharmaceutically acceptable, 71
innocuous, nonionic biological surfactant.29, 30
Innocuous PEG-based surfactants show 72
high selectivity in disrupting vesicular membranes.31-33
Such a vesicular system offers 73
an unique molecular compartmentalization to make it a better vehicle to carry out as 74
well as to modulate different types of chemical reactions and their application serving 75
as an efficient mimetic system. 76
Following conventional ET theory, as originally developed classically by Marcus and 77
thereafter undergone many modifications incorporating the quantum mechanical 78
aspects into it, 2,4,34-37
the rate of an ET reaction can be expressed as 79
80
where is the Planck constant divided by 2 , Vel is the electronic-coupling matrix 81
element, kB is the Boltzmann constant, T is the absolute temperature, is the free 82
energy of the reaction, and is the total reorganization energy, which is the sum of 83
two reorganization energies as 84
= (2) 85
Where, is the intramolecular reorganization energy and is the solvent 86
reorganization energy. The most important prediction that emerges from Eq.1 is the 87
inversion of the ET rate at exergonicities ( ) higher than , a phenomenon 88
commonly known as Marcus inversion. According to Eq.1, in the normal region 89
< ) kET should increase with ), but it should reach its maximum value at 90
the barrierless condition of = , and thereafter in the region of ( < ) the 91
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kET should decrease with − , giving the expected inverted region. The concept of 92
Marcus inversion was a controversy for quite a long time due to the lack of 93
experimental evidences for such typical behavior in the ET rates. Apparently Marcus 94
inversion is easier to observe for bimolecular ET reactions in microheterogeneous 95
media than for those in homogeneous solutions and this preference is due to favorable 96
situation provided by the topology of the former in relation to slow relaxation and 97
reduced diffusion of the reactants. At present, however, a large number of 98
experimental results have demonstrated Marcus inversion in the ET reactions, mostly 99
in intramolecular ET processes, where the reacting donor and acceptor moieties are 100
chemically bound to each other,38-42
and in back ET (BET) reactions in radical ion-101
pairs, where the reacting species are in physical contact. In bimolecular ET reactions 102
under diffusive conditions, there are two main constraints that can obscure the Marcus 103
inversion.3,43-49
They are (i) the diffusion of reactants, which limits the bimolecular 104
reaction-rate constant not to exceed the diffusional rate constant kd, and (ii) the 105
difficulty in finding suitable homologous series of the donors and/or acceptors such 106
that the reaction exergonicity can be varied over a large range, especially the higher-107
exergonicity region, where the ET rates can eventually become lower than the 108
diffusion-controlled limit kd. Due to the above limitations, only a few examples are 109
reported in the literature to show clear Marcus inversion behavior in bimolecular ET 110
reactions. 111
Previous works on ET reaction inside vesicular system have shown that charge 112
separation and radical ion yield are modulated by various factors such as surface 113
charge, headgroup variation, and acyl chain length variation19, 50,51
. So far our group 114
has reported a lot of works regarding PET inside various confined media 21,52,53
In this 115
communication we are interested to know whether the trivial ET reaction between 116
Coumarin dyes and DMA in this biologically relevant organized system will show 117
inversion in vs lnkq correlation curves according to Marcus theory or not. 118
Additionally we have tried to check whether ET reaction in such type of 119
heterogeneous media is diffusion guided or not. Recently ET reaction inside liposome 120
of small unilamellar vesicle (SUV) type has been investigated, from which a high ET 121
reaction rate for all the common coumarin dyes (acceptors) showing an inversion has 122
been observed. In this study, special attention has been paid to the unusually high 123
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values of the rate constant of Coumarin-152 and Coumarin-152A.15f
They have 124
explained this unusual fact by considering the participation of the twisted 125
intramolecular charge transfer (TICT) states of these two dyes in ET kinetics. The 126
population probability of this TICT state depends on the electron donor-acceptor 127
capacities of the involved partners and on the solvent polarity which would stabilize 128
the highly polar structure. Furthermore, solvent viscosity could also affect the 129
nonradiative deactivation since the process involves a rotational motion of the bond 130
linking the partners. On the other hand, some authors have suggested that the specific 131
hydrogen bonding between solvent and electron donor group also plays a major role to 132
stabilize the twist conformer to facilitate the formation of the TICT state54a-c
. The role 133
of the hydrogen bonding of the electron acceptor with solvent in the formation of the 134
TICT state is an important subject in exploring the proton-coupled charge transfer 135
phenomena often observed in biological assemblies.54d
In our vesicular system we also 136
want to investigate the effect of participation of TICT state on ET rate of these two 137
dyes. 138
139
2. Experimental section. 140
The Coumarin dyes were obtained from Exciton (laser grade) and used as received. N, 141
N-dimethylaniline (DMA), PEG-6000, and Tween-80 were obtained from Aldrich 142
chemical and DMA was distilled under reduced pressure before use. Experiments 143
were carried out at 298(±1) K. The chemical structures of Coumarin dyes, DMA, and 144
Tween-80 are shown in Scheme 1. Niosome has prepared as described by sonication 145
method.26a,b
Briefly Tween 80, PEG-6000 and water were vortex mixed at a certain 146
molar fraction to prepare the lamellar liquid crystal and then niosome was obtained by 147
sonicating the diluted solution (2% PEG+98% water) with the lamellar liquid crystal 148
for 30 ~85 min. The final concentrations of all acceptor Coumarin molecules in the 149
experiment were kept 5x10- 6 M. 150
The absorption and fluorescence spectra were measured using a Shimadzu (Model no: 151
UV-1601) spectrophotometer and a Jobin Yvon Fluoromax-3 spectrofluorimeter. The 152
fluorescence spectra were corrected for the spectral sensitivity of the instrument. For 153
steady state experiments, all samples were excited at 408 nm. The time resolved 154
fluorescence setup is described in detail in our earlier publication21
. Briefly, the 155
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samples were excited at 408 nm using a picosecond diode laser (IBH, Nanoled), and 156
the signals were collected at magic angle (54.70) using a Hamamatsu microchannel 157
plate photomultiplier tube (3809U). The instrument response function of our setup is 158
110 ps. The same setup was used for anisotropy measurements. The analysis of the 159
data was done using IBH DAS, version 6, and decay analysis software. The same 160
software was also used to analyze the anisotropy data. 161
162
3. Results: 163
3.1. Steady State absorption and emission spectra 164
We have checked the emission and absorption spectra of all the coumarins in several 165
solvents, such as acetonitrile, ethyl alcohol, 50% ethyl alcohol/water (v/v), methanol 166
etc. to get an idea about the polarity of the microenvironment sensed by the dyes. Final 167
results matched quite well with those obtained in ethyl alcohol. The absorption and 168
emission peaks of all Coumarin dyes inside niosome and in 50% ethyl alcohol/water 169
are listed in Table 1. The steady state fluorescence quenching of coumarins with 170
addition of DMA has been shown in Fig. 1. All the absorption spectra were broad and 171
structureless, comparable to their spectra in water, but these were blue shifted 172
compared to water. In an analogous fashion, emission spectra of the coumarins were 173
also broad and structureless, bearing similarities to their nature in water, but these 174
were again blue shifted compared to water. Moreover we have observed no exiplex 175
formation as there is no change in nature of the emission spectra of the coumarin dyes 176
after addition of quencher. Considerable deviation from linearity was observed for 177
steady state Stern-Volmer plot (SV) 54
and the necessary equation for SV plot is given 178
in the forthcoming section. 179
180
3.2. Determination of Distribution coefficient. 181
We have calculated the distribution coefficient (KD) of coumarin dyes between the 182
niosome and water continuous phase by method of UV-vis difference spectra.28a
183
184
185
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Where, CS is the surfactant concentration. Eψ is the apparent mole absorption 186
coefficient of a particular dye at a given wavelength. Ew and Em are the apparent mole 187
absorption coefficients of the dye in water continuous phase and in niosome, 188
respectively. Eψ and Ew can be determined experimentally. So, by plotting 1/ (E ψ - Ew) 189
against 1/CS , the distribution coefficient KD can be determined from the slope (Figure 190
3 ). For all the dyes we have obtained quite high values of distribution coefficient ~104 191
at 250C. 192
193
3.3. Time-Resolved lifetime measurements 194
For quantitative investigation of quenching rates inside niosome, time resolved 195
spectroscopy was carried out in presence and in absence of quencher. The time 196
resolved fluorescence decays were taken at the emission maxima of the coumarin dyes 197
in niosome. Single exponential decay in absence of DMA is observed for all the 198
coumarin dyes except C-152 and C-152A. For these two dyes, decay curves are 199
biexponential in nature. But after addition of quencher, all the dyes showed 200
biexponential decay. The average lifetime of all the dyes inside niosome are given in 201
Table 3 and decays are shown in Figure 3 where average lifetime is calculated by 202
using the following equation. 203
204
Where , are the first, second and third component of decay time constants of the 205
coumarins and , are the corresponding relative weightage of these components 206
respectively. 207
Electron transfer rate constant (kq) has been calculated by using Stern-Volmer 208
equation55
209
210
Where, and are the fluorescence lifetime of the Coumarin dyes in absence and in 211
presence of quencher. The typical plot for / vs. [Q] is shown in Fig. 4 and the 212
values obtained from time resolved studies are listed in Table 4 and is time 213
resolved SV constant, where . All the SV plots show linearity and from 214
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the slope we have calculated the time-resolve ET rate constants for all the Coumarin-215
DMA pairs inside niosome. In this study we have taken the actual concentration of 216
quencher to determine instead of effective concentration as done in case of small 217
micellar systems. Although DMA solubilizes quite selectively in the bilayer region it 218
is not possible to know the true DMA concentration at the solubilization sites of the 219
coumarin dyes inside multilamellar structure of niosome having thickness of 80-220Å. 220
Furthermore multilamellar niosome structure is polydisperse in nature with their 221
diameter varying from 1000-1500 Å.28b
So the rate constants are relative in this study 222
and these should not be compared with the corresponding rate constants obtained for 223
similar donor acceptor system in micelles or reverse micelles because the effective 224
amine concentration inside such confined media is largely dependent on the nature of 225
such microheterogeneous media. But our results will be useful information to compare 226
results obtained in such large multilamellar membrane bioassemblies that are 227
frequently found in natural biological systems and also to predict the dependency of 228
the rate constant with the free energy changes, especially to make a correlation with 229
the Marcus ET theory. 230
231
3.4. Time-Resolved anisotropy measurements 232
To get an intimate idea regarding the location of the probe i.e., microenvironment 233
around the dyes inside niosome, we have performed time-resolved fluorescence 234
anisotropy study. The time-resolved fluorescence anisotropy (r (t)) is obtained by 235
using the following equation 236
237
Where, G is the correction factor for detector sensitivity to the polarization direction 238
of the emission. The value of G factor in our case is 0.6. I║ (t) and I⊥(t) are 239
fluorescence decays polarized parallel and perpendicular to the polarization of the 240
excitation light, respectively. The fitted results for all the anisotropy decays are listed 241
in the Table 2. All the anisotropy decays for the dyes were found to be biexponential 242
in nature inside niosome. The average anisotropy was calculated by using the 243
following equation 244
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245
Where represents average anisotropy, and , represents fast and slow 246
component and , are their relative components. We have determined the 247
microviscosity (η) inside Niosome following the well known Stokes-Einstein-Debye 248
relation (equation 8) 249
250
Where, V is the volume of the fluorophore, kB is the Boltzmann constant, and T is the 251
absolute temperature. The microviscosity obtained inside niosome is quite large ~ 45 252
cp compared to bulk water viscosity (~ 1 cP), even larger than the viscous solvent 253
ethylene glycol (~13.5 cP). 254
255
3.5. Free Energy Calculation: 256
To have quantitative idea about ET rate and its dependency on free energy change 257
( G0) we have to calculate the change in free energy for all the coumarin-DMA pairs 258
present in niosome. The usual expression to calculate is given by the famous 259
Rehm–Weller equation, which we have discussed in our earlier publications21,52,53
260
261
(9) 262
Where, E00 is the energy required for the transition of coumarin dyes from ground 263
state (S0) to first excited electronic state (S1). This was obtained from the intersection 264
point of normalized absorption and emission spectra and r denotes the centre to centre 265
contact distance between coumarin and DMA. The static dielectric constant of the 266
medium is described by where e is the charge of an electron. Here E (D/ D+) and E 267
(A/A-) denote the oxidation potential of the donor and reduction potential of the 268
acceptor. Since the emission and absorption behavior of all the coumarin dyes matches 269
most closely with the values that in 50% ethyl alcohol/water, so all the measurements 270
of the reduction and oxidation potential have been carried out in ethyl alcohol. The 271
results thus obtained are shown in table 4. The radii of the molecules were calculated 272
using Edward’s volume addition method.56
273
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4. Discussion. 274
A close look into the steady state absorption and emission spectra can give us some 275
confirmative inferences about the location and behavior of the various coumarin dyes 276
(acceptor) inside the niosome. From the blue shifted absorption spectra of all the 277
coumarin dyes compared to those in water and good agreement with the absorption 278
maxima in 50% ethyl alcohol/water, one can arrive at the conclusion that all the dyes 279
are now transferred from polar bulk water to niosome, comparatively a less polar 280
region. Again, blue shifted emission spectra for all the dyes also indicate a less polar 281
microenvironment around all the dyes inside niosome. The solubility of the dyes also 282
increases in niosome-containing solution compared to the very low values in bulk 283
water. Moreover the emission maxima and absorption maxima for all the dyes are 284
greater than that in pure water and lower than that in cyclohexane but well matched 285
with the values obtained in 50% ethyl alcohol/water. These results provide evidence to 286
support the idea of solubilization of the dyes in such a region which have polarity 287
greater than cyclohexane ( 2.03) [Hydrocarbon alkyl chain of surfactant has very 288
low polarity comparable with the polarity of highly nonpolar solvents such as 289
cyclohexane] but less than polarity of water ( 80.1) but similar to a 290
microenvironment having polarity resemble with 50 % ethyl alcohol/water ( ) 291
i.e., headgroup like region which have polarity greater than hydrocarbon chain but less 292
than water pool. All inferences about solubilization sites again were confirmed by 293
determination of distribution coefficient of all the dyes. We have obtained large values 294
of KD for all the dyes (~ 104) inside niosome (table 1). Thus, the preceding discussion 295
points to the confinement of the coumarin dyes inside niosome headgroup region 296
rather in bulk water. Here we have depicted pictorially the structure of niosome 297
(scheme 1) in which one can see that there is region where two layers of headgroup 298
are present, and we have defined this region as bilayer headgroup region. This bilayer 299
headgroup region in our niosome system is special because in this region, PEG-6000 300
is squeezed from two sides by surfactant headgroups. PEG-6000 is also adsorbed on 301
the two extreme headgroup regions and forms a thick water layer around the 302
headgroup region. Now it can be said that the coumarin dyes are distributed over both 303
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type of headgroup regions and both the regions are very similar with respect to 304
polarity but differ a little with respect to rigidity. 305
Although there is no proof of formation of ground state complexation, the positive 306
deviation of SV plot from linearity is highly suggestive of accumulation of DMA near 307
to the location of the acceptor coumarin molecules. The concentration of DMA inside 308
niosome is expected to be high as it is a hydrophobic molecule and this should be near 309
the acceptor molecules as ET occurs when donor-acceptor are in contact to each other. 310
So the presence of high concentration of DMA in physical contact with acceptor prior 311
to the excitation causes instantaneous static quenching in SV plot. But the linear SV 312
plot obtained from time resolved study indicates that the dynamic quenching is 313
measured by this technique 15a-i, 16a-c
. In our study we have used kq values obtained 314
from time-resolved SV results as given in Table 4 and correlated with figure 5. ET 315
rates for all the coumarin-DMA pairs are of high values compared to many other 316
nano-size confined media20,52,53,15a-f,16a,b
. C-152 and C-152A-DMA pairs show higher 317
values compared to other pairs in our study. Rate constants of such high magnitude 318
have been observed in lipid vesicle system where C-152 and C-152A-DMA pairs also 319
have unusually higher rate constants compared to other pairs15f
. 320
To understand more comprehensively the microenvironment experienced by the 321
acceptor coumarin molecules inside niosome we have carried out anisotropy decay of 322
all the coumarin dyes. Large values of rotational relaxation time constants for all the 323
coumarin dyes inside niosome (Table 2) compared to that of in bulk water (~100 ps) 324
surely indicate the confinedness of the probes inside niosome bilayer region. C-151 325
have greater value of anisotropy time constants over all the coumarin dyes and it is 326
due to probable formation of H-bonding between the amino group of C-151 and 327
oxyethyelene group of bilayer headgroup region. Fluorescence anisotropy time 328
constants in neat PEGs (~1.8 ns) 57
are slightly large compared to that in niosome. 329
This is because of higher viscosity of that neat solution of PEGs. 330
The appearance of Marcus inverted region in the lnkq vs plot for the coumarin-331
DMA pairs inside niosome is the most interesting feature of this work. It was 332
previously proposed that the quenching rate constant does not depend on the diffusion 333
of the reactants in large confined media like lipid vesicle15f
and also in small confined 334
media like micelle 15b,d 16a-c,
where the reactants remain effectively static during the 335
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course of the reaction. If the reactants are tumbling freely in solution, i.e., rapid 336
diffusion takes place then the quenching process will be diffusion-controlled in higher 337
exergonicity region. As a consequence the rate constant values will reach a saturation 338
at the bimolecular diffusion controlled limit (kd) at higher exergonicity instead of 339
showing Marcus inverted region as observed in many studies.15,58a,b,59a-e
. But in our 340
multilamellar vesicle system, molecular diffusion inside should not be so fast that it 341
exceeds the time scale of ET reactions. In this work the high limit of electron transfer 342
rate kq is in the order of 10.0 x 1010
dm3.mol
-1.s
-1. To observe the Marcus inverted 343
region the lower limit of kd value should be leveled with kq values. Thus the kd value 344
for coumarin-amine pair should be at least ~10.0 x 1010
dm3.mol
-1.s
-1. The 345
microviscosity inside niosome is very high (45 cP) compared to acetonitrile solution 346
(0.35 cP) and the reported bimolecular diffusion controlled rate constant for coumarin-347
amine pair in acetonitrile solution is 1.5 x 1010
dm3.mol
-1.s
-1. Average anisotropy of C-348
153 in our system is 50 times larger compared to the anisotropy value in acetonitrile 349
solution. Moreover anisotropy value for the same probe is 250 ps in butanol having 350
microviscosity of 2.74 cP11,60a,b
. Comparing the above values regarding 351
microviscosities, one can expect that the kd value in our niosome system is 1.46 x 108 352
dm3.mol
-1.s
-1(assuming η = 45 cP) which is much lower than the higher value of kq 353
observed in present system. Pal et al also have obtained kd values in the order of 1x 354
108 dm
3.mol
-1.s
-1 in lipid vesicle which was lower than their observed ET rate and they 355
have concluded that diffusion of the reactants did not play any significant role in 356
determining the quenching kinetics15f
. Such non-diffusive nature of ET rate was also 357
observed in other complex heterogeneous media such as micelles, reverse micelles and 358
the quenching kinetics was attributed to the spatial distribution of the reactants.15a-f,58a,b
359
Again the observation of non-single exponential fluorescence decay of coumarins in 360
presence of DMA also supports the assumptions of negligible contribution of diffusion 361
on the quenching kinetics in our system. Under diffusive condition it has been 362
reported that the fluorescence decay follows the single exponential nature even in 363
presence of donor molecules.17,61a,b
In this work, as there is a distribution in the 364
coumarin-DMA separations, the observed quenching kinetics will be dominated by the 365
effective distribution of the quenchers around the excited coumarin dyes inside 366
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niosome headgroup region. A Similar observation has also been shown by various 367
experimental as well as theoretical studies.19,62
. 368
From figure 6 we can clearly see that the maxima in the Marcus correlation curve 369
occur at an exergonicity of ~ 0.62-0.67 eV, which is slightly lower than the value 370
expected from the consideration of solvent reorganization energy. Similar observation 371
was obtained in various micellar assemblies like sodium dodecyl sulfate (SDS), triton-372
100 (TX-100) but in case of cetyl trimethylammonium bromide (CTAB) and dodecyl 373
trimethylammonium bromide (DTAB) micelles the maxima appeared at ~ 1.2 eV15c,g,h
. 374
In our system we have calculated solvent reorganization energy (λs) as ~ 1.1 eV 375
considering the average values for rD, rA, and rDA (3.05 Å, 3.68 Å, and 6.73 Å) by 376
using the following Marcus derived equation for λs2,3,9,39,50 377
378
Where e is the electronic charge, n is the refractive index and is the dielectric 379
constant of the medium. So the total reorganization energy obviously greater than 380
. Solvation rates in CTAB and DTAB micelles are much faster than ET 381
rates but in case of SDS and TX100 micelles solvation rates are slower or comparable 382
with the ET rates. Our study matches well with the results obtained inside SDS type 383
micelle and small unilamellar vesicle as reported previously15f
. Marcus inversion at 384
the lower exergonicity region is observed here and solvation rate is much slower than 385
ET rate. Due to slow solvent dynamics in various microheterogeneous media, the 386
effective solvent reorganization energy for the ET reactions is lower than that in 387
homogeneous solvents with fast solvent dynamics. This lowering in effective solvent 388
reorganization energy is due to the applicability of the two dimensional ET (2DET) 389
theory15a-e,11a-c
inside niosome because in this situation the reactant state of ET system 390
cannot attend the equilibrium solvent configuration during ET time. Thus, 391
conventional ET theory is not applicable for the reactions inside niosome system. On 392
the basis of this theory, ET reaction effectively takes place along the intramolecular 393
coordinate axis (q) even when the equilibration of the reactant state along the solvent 394
coordinate axis (X) is incomplete. The average solvation time at 250C is 1583 ps inside 395
niosome63
[at the same concentration of tween80 and PEG-6000 as used in this study, 396
ex=410 nm] which is very slow compared to ET rates. We have used C-153 as our 397
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solvation probe which has emission maxima at 526 nm inside niosome and it shows 7 398
nm red-shift upon change in ex from 375 nm to 440 nm. In many studies it was 399
reported that poly (oxyethyelene) group can show ice-like structure which is very 400
constrained and contained a highly dense hydrogen bonded water network64
and this 401
surprisingly ultraslow relaxation kinetics of water molecules around the solvated dye 402
embedded inside the bilayer headgroup region is attributed to the presence of a strong 403
H-bonding environment of water molecules in the headgroup region, and movement of 404
these highly bound water molecules along with a hydrated oxyethyelene moiety 405
control the observed slow relaxation. Presence of such site specific solvation dynamics 406
of coumarin dyes inside highly dense oxyethyelene environment may be influenced by 407
the formation of excited-state intermolecular hydrogen bonding between solvents and 408
coumarins inside niosome65
. Such slow solvation comparable to ET rates indicates that 409
ET reaction in multilamellar niosome is qualitatively very similar to those in other 410
heterogeneous media such as micelles and reverse micelles. Steady state absorption 411
and fluorescence spectra shows similarities between the spectra obtained for neat 412
PEGs57, 58
and this is possible only when the dyes sense the presence of PEG 413
molecules inside niosome very well because in the former one PEG is in neat solvent 414
and in the later, PEG is entrapped inside a confined media. Previous works in neat 415
PEGs shows that solvation dynamics of PEG in neat PEG solutions is quite faster 416
(~600 ps) 57,66
compared to water dynamics in our niosome system and it is quite 417
difficult to evaluate the PEG dynamics in our complex niosome system. But we can 418
compare the PEG solvation dynamics (~600 ps) in neat PEGs with our ET time 419
constants which are quite fast (~200 ps). It will be very speculative for us to ascertain 420
individually about the competitiveness between the PET rate with the water dynamics 421
as well as PEG dynamics inside niosome system. 422
In this article we have shown that diffusion has insignificant contribution to the ET 423
kinetics. Moreover, conclusions drawn by many authors regarding the effect of lateral 424
diffusion on ET kinetics in such vesicle system and many organized media allows us 425
to assume that similar phenomenon is happening here i.e., non-diffusive ET14, 15f,16a-c
. 426
As both the donors and acceptors are π-type in character the role of rotational 427
diffusion can also be neglected. Using ultrafast component of ET dynamics inside 428
micelles and cyclodextrin, Bhattacharyya et al have shown that diffusion have no 429
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15
effect on ET rates15a-c
. We have already observed that the high distribution coefficient 430
values for all the dyes inside niosome compared to bulk water and the absorption and 431
emission spectra for all the dyes are well suited with the results obtained for ethyl 432
alcohol solution. Accordingly we can say that all the acceptors are located in the 433
headgroup bilayer region of the niosome. But there is very much possibility of H-434
bonding interaction of free –NH2 group of C-151 and oxygen of highly dense 435
oxyethyelene groups presents in the bilayer headgroup region and its rotational 436
relaxation value is slightly higher than to the values of a hydrophobic non H-bond 437
forming probe (C-153). However, since the thickness of the bilayer is quite large (~8-438
20 nm) 28b
compared to the dimension of the coumarin dyes (~ 1 nm) we do not expect 439
that the hydrogen bonding interaction of C-151 dye with the oxyethyelene groups 440
presents in the bilayer headgroup will cause any significant change in the localization 441
site of the dye compared to that of the other dyes. This is also indicated by the large 442
value of distribution coefficients from bulk water to niosome for all the coumarin 443
dyes. So there is hardly a chance for any difference in location of the probes. Thus the 444
differences in ET rates for all the pairs are mainly due to difference in energetics 445
( ) of the ET reactions. ET reactions inside P123triblock copolymer micelles15a
, 446
small unilamellar vesicles15f
and inside reverse micelles 15e
and its correlation with 447
free energy have shown that not only C-151 but also other dyes fall in the Marcus 448
inversion region. These results help to draw the conclusion that Marcus inversion 449
observed in the present study is a result of differences in energetics of the ET 450
reactions. A distinguishing feature is observed in case of ET dynamics of two 451
acceptors C-152 and C-152A in niosome assembly. These dyes have very high ET 452
rates compared to the other coumarins under similar photophysical condition. Such 453
unusual behavior of these dyes was also observed inside unilamellar lipid vesicles15f
. 454
To understand the inner story behind their large kq values in niosome we have to 455
consider various possibilities that can happen in the excited state of these two dyes 456
under the given microenvironment by niosome. 457
Firstly, we have to focus on the results from the time resolved study of these two dyes 458
inside niosome. These dyes show biexponential decay in absence of donor while other 459
acceptors show single exponential decay. Their different location should not be the 460
reason for their biexponential nature because single exponential nature of other dyes 461
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16
does not support it. One tentative conclusion can be arrived by considering the 462
structural and photophysical properties of these two dyes. These two dyes have no free 463
hydrogen at the amino group and can’t form H-bond with the oxyethyelene moiety 464
inside niosome. Moreover anisotropy values of these two dyes are very similar 465
indicating similar microenvironment around both the dyes inside niosome and these 466
two have flexible N, N-dialkyl groups. Thus it is expected that these two can undergo 467
intramolecular charge transfer (ICT) to twisted intramolecular charge transfer (TICT) 468
in their excited state as described in literature67a-d
.Many previous studies have already 469
reported such TICT state formation by C-152 and C-152A in polar solvents but at the 470
same time C-151 is not assigned as a candidate for such behavior.67a-d
This is mainly 471
due to lower electron donor ability of –NH2 group than N, N-dialkyl group. Structural 472
similarities of these two dyes imply that they should show identical behavior in 473
relation to the TICT state formation. It has already been reported such TICT state 474
formation of these two dyes even in an intermediate solvent polarity region67a
. In our 475
system we have assumed the polarity of niosome headgroup part is very close to the 476
polarity of 50% ethyl alcohol/water, which is sufficient for showing such type of TICT 477
state. Moreover, excited state hydrogen bonding of the electron acceptor group of a 478
compound (which can show ICT-TICT) with the polar solvent may also directly 479
influence the stability of the ICT and TICT states and such effect was previously 480
demonstrated in theoretical calculations68
. As we have mentioned that the bilayer 481
headgroup region of niosome is highly hydrated (dielectric constant also high, 37.1), 482
so strong hydrogen bonding between carbonyl oxygen and water molecules is present 483
in our system. It is very much possible that the >C=O۰۰۰H۰۰۰OH bond is a key 484
governing factor of the dynamical exchange between the two states. Recently it was 485
first reported by Zhao and Han68
that the non-emissive and emissive ICT states can be 486
largely modulated by the intermolecular hydrogen bond which is present between 487
hydrogen bonding solvent and the compound compared to non-hydrogen bonding 488
solvent. It was also reported TICT state formation by these dyes inside small 489
unilamellar vesicle of comparatively low polarity with respect to our system.15f
Like us 490
they have also obtained biexponential lifetime decay of these two dyes inside small 491
unilamellar vesicle, while other studies revealed that these dyes show only single 492
exponential nature in micellar media52,15b,d,g,h
in absence of DMA implying that a 493
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different type of microenvironment is provided by our niosome system in comparison 494
to normal micellar system. Moreover Pal et al. has described the presence of TICT 495
states by considering the faster rotational relaxation of these dyes comprising of 496
twisting dynamics in comparison to other dyes15f
. This discussion can again be 497
elaborated by considering the high microviscosity sensed by our coumarin probes 498
inside niosome. In normal low viscous solvents the non-emissive TICT states of these 499
two coumarins undergo fast nonradiative decay, making very short lived TICT states. 500
Since their inter-conversions are also fast, so there is hardly any chance of the reverse 501
process from TICT to ICT state. In our niosome system we got high microviscosity of 502
the region where the coumarins are located. Such larger viscosity (~45 cp) hindered 503
the fast decay process of TICT state making them comparatively long lived. Although 504
higher viscosity retarded the interconversion from ICT to TICT states, it is now 505
sufficient for participation of the TICT state of these two dyes as evident from their 506
fast decay lifetime inside niosome. As the lifetime of TICT state is considerably high 507
compared to its emissive ICT state inside high viscous niosome bilayer headgroup 508
region, so it is expected that this TICT state also interacts with the donor molecules 509
during quenching process. The reversibility of these two states allow the conversion 510
from fluorescent ICT state to non-fluorescent TICT state as more TICT states get 511
quenched during interaction with DMA. With addition to viscosity and polarity effect 512
on stabilization of the highly polar non-emissive TICT states of these two dyes, 513
intermolecular hydrogen bonding between the polar TICT states and polar protic 514
solvent (here water) also makes it long lived. As the dipolar character is greater in 515
TICT states compared to the ICT states, it is expected that intermolecular hydrogen 516
bonding effect will be more prominent in case of TICT states as demonstrated by Zhao 517
and Han.69
It is now very clear that the emissive ICT state is diminished 518
simultaneously by two paths during ET takes place. So there will be an additional 519
channel for quenching for these two dyes during ET reactions, which increase their 520
quenching rate compare to other coumarin dyes. 521
Lastly we can summarized that ET reaction inside niosome is not diffusion controlled 522
like other organized media14,15f,16a-c
and energetics of ET reaction in such 523
heterogeneous bioassemblies is the key factor for appearance of bell-shaped Marcus 524
inversion region. ET kinetics is mainly determined by various spatial distribution of 525
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DMA around the excited coumarin dyes. Chemical timing effect is also a significant 526
character in such two dimensional ET reactions. Slow solvation dynamics compared to 527
ET rate shifts the inversion maxima to the lower exergonicity. Moreover unusual high 528
ET rates of C-152 and C-152A can be attributed due to presence of non-emissive 529
TICT state which interacts simultaneously with the quenchers along with the emissive 530
ICT state. As a consequence, ET kinetics become fast by the participation of an 531
additional intramolecular dexcitation process. 532
533
Conclusion. 534
Intermolecular electron transfer between N, N-dimethylaniline as donor and various 535
coumarin dyes as acceptor has been performed inside a large bioassemblies, niosome. 536
Location of the dyes inside niosome was verified and quenching of fluorescence 537
intensity was found to be biexponential in nature for all coumarin dyes. Comparing the 538
results inside niosome with various other nanosized confined media it is clear that ET 539
reaction inside niosome is not diffusion guided; rather the ET kinetics is governed by 540
the spatial distribution of donor and acceptor inside niosome. A bell shaped 541
Marcus inversion with a maximum at lower exergonicity is observed. Such shifting of 542
maxima comparable to expected value is mainly due to presence of slower solvation 543
dynamics. 2DET theory was used to explain the effect of nonequilibrium solvent 544
excited state on ET rates. Inside niosome we have obtained high ET rates for C-152 545
and C-152A, which is modulated by the presence of stable TICT state along with 546
normal ICT state. 547
548
Acknowledgement: 549
N.S. is thankful to the Board of Research in Nuclear Sciences (BRNS), Council of 550
Scientific and Industrial Research (CSIR), Government of India for generous research 551
grants. C. G, V. G. R, S.M. are thankful to CSIR for research fellowships. We 552
sincerely acknowledge the help of Mr. Shirsendu Ghosh during execution of some of 553
the experiments. 554
555
556
557
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699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
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716
717
718
Table.1. Absorption and emission maxima of different Coumarin dyes 719
720
721
722
723
724
Table.2 Rotational relaxation parameters of the Coumarin molecules inside Niosome 725
726
727
728
729
730
731
732
733
734
Coumarins
abs(nm) em(nm) Distribution
Coefficient (250C)
50 %
Ethanol
Niosome Water Ethanol Niosome Water
C-151 382 383 364 480 483 495 5.7 x 104
C-152 397 399 402 510 509 526 8.7 x 104
C152A 405 407 411 509 507 528 2.8 x104
C-480 389 390 392 465 468 489 12.0 x 104
C-153 423 425 434 530 526 549 5.8 x 104
Coumarin r0 a1r a2r τ1r
(ns)
τ2r
(ns)
<τr>
(ns)
C-151 0.38 0.58 0.42 0.82 3.06 1.76
C-152 0.36 0.40 0.60 0.57 1.48 1.11
C152A 0.39 0.43 0.57 0.52 1.37 1.01
C-480 0.37 0.84 0.16 0.65 5.66 1.31
C-153 0.38 0.76 0.24 0.84 3.62 1.38
Electronic Supplementary Material (ESI) for Physical Chemistry Chemical PhysicsThis journal is © The Owner Societies 2012
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735
Table. 3.Decay parameters of the Coumarin dyes inside Niosome in absence of DMA 736
and in presence of DMA. 737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
Table. 4 Lifetime, quenching constants, redox potentials, E00 values and G0
for 755
different Coumarin-amine systems studied inside Niosome. 756
757
System TR kq/1010
(M-1
S-1
)
E(A/A-)V
/vs. SCE
E00
(eV)
E(D/D+)V /vs.
SCE
G0(eV)
C-151 3.90 1.53 2.698 0.711 -0.889
C-152 8.20 1.58 2.75 -0.610
C152A 9.06 1.64 2.71 -0.514
C-480 1.79 2.00 2.83 -0.269
C-153 4.20 1.66 2.56 -0.339
Acceptor
(Coumarin)
Donor
(DMA)
(mM)
a1
1
(ns)
a2
2
(ns)
<τav>
(ns)
C-480 0 1.00 5.78 — — 5.78
C-480 39.4 0.64 1.17 0.36 3.40 1.97
C-153 0 1.00 4.27 — — 4.27
C-153 39.4 0.70 0.48 0.30 1.78 0.65
C-152A 0 0.88 1.08 0.12 2.03 1.19
C-152A 39.4 0.65 0.14 0.35 0.59 0.30
C-152 0 0.94 1.09 0.06 2.50 1.16
C-152 39.4 0.63 0.26 0.37 0.74 0.43
C-151 0 1.00 5.90 — — 5.90
C-151 39.4 0.91 0.86 0.09 3.56 1.09
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26
758
759
760
761
762
763
Coumarin 151 (C-151) Coumarin 152 (C-152) Coumarin 152A (C-152A) 764
765
766 767
768
769
770
Coumarin 153 (C-153) Coumarin 480 (C-480) Dimethyl aniline (DMA) 771
772
773
774 775 776 777 778 779 780 781 782 783 784
Scheme 1: Structures of Coumarin dyes, N, N-dimethylaniline, Tween80, and Two 785
dimensional representation of multilamellar noisome. 786
787
O O
CF3
NH2
O O
CF3
(CH3)2N
O O
CF3
(C2H
5)2N
O O
CF3
N
O O
CH3
N
NMe2
HOO
O
O
O
O
OH
OH
O
O
W+X+Y+Z=20
Y
X
W
Z
Tween 80
BilayerHeadgroupRegion
Electronic Supplementary Material (ESI) for Physical Chemistry Chemical PhysicsThis journal is © The Owner Societies 2012
27
788
789
790
791
792
793
794
795
796
Fig. 1 The Steady state absorption spectra (dotted line) and fluorescence emission 797
spectra (dashed line) of C-153 inside niosome and in water (solid line) 798
799
800
801
802
803
804
805
806
807
808
809
Fig. 2 The Steady state fluorescence quenching spectra of (a) C-151, (b) C-152, (c) C-810
480, (d) C-153 at bulk DMA concentrations: (i) 0 mM (ii) 7.88 mM, (iii) 15.76 mM 811
(iv) 23.64 mM, (v) 31.52 mM, (vi) 39.4 mM, (vii) 47.28 mM.812
(b)
(vii)
(i)
(vii)
(i) Addition of DMA (l)(a)
(vii)
(i)
Addition of DMA (l)
(vii)
(i)
Flu
orescen
ce I
nte
nsit
y
450 500 550 600 650 700
(d)
(vii)
(i)
Wavelength (nm)
(vii)
(i)
Addition of DMA (l)
450 500 550 600
(c)
Wavelength (nm)
Flu
orescen
ce I
nte
nsit
y
Addition of DMA (l)
(vii)
(i)
(vii)
(i)
400 450 500 550 600 650 7000.0
0.2
0.4
0.6
0.8
1.0
Wavelength (nm)
No
rma
lis
ed
In
ten
sit
y
Electronic Supplementary Material (ESI) for Physical Chemistry Chemical PhysicsThis journal is © The Owner Societies 2012
28
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
Fig. 3 The time resolved fluorescence quenching decay of (a) C151, (b) C-152, (c) C-828
480, and (d) C-152A with bulk DMA concentrations: (i) 0 mM (ii) 7.88 mM, (iii) 829
15.76 mM (iv) 23.64 mM, (v) 31.52 mM, (vi) 39.4 mM. 830
831
832
833
834
835
836
0 4 8 12 16 200
1000
2000
3000
4000
5000
Time (ns)
Co
un
ts
(a)Addition of DMA
(vi)
(i)
(vi)
(i)
0 2 4 6 80
1000
2000
3000
4000
5000(b)
(i)
(vi)
(vi)
(i)
Addition of DMA
Time (ns)
Co
un
ts
0 4 8 12 16 200
1000
2000
3000
4000
5000(c)
Addition of DMA
(vi)
(i)
(vi)
(i)
Cou
nts
Time (ns)0 2 4 6 8
0
1000
2000
3000
4000
5000
(i)
(vi)
(vi)
(i)
(d)
Addition of DMA
Time (ns)
Cou
nts
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837
838
839
840
841
842
843
844
845
Fig. 4 Plotting of 1/(E ψ- EW) against 1/CS-1
, where (●)= C-480, (♦)= C-153, 846 (▲)=C-152, (■)= C-152A, (▼) =C-151 847 848
849
850
851
852
853
854
855
856
857
858
859
Fig. 5 Time resolved Stern-Volmer plots for (●) = C-480, (♦)= C-153, (▲)=C-152, 860
(■)= C-152A, (▼) =C-151. 861
862
50000 100000 150000 200000 250000
0
10
20
30
40
CS
-1 (l mol
-1)
(E-E
W)-1
x 1
05 /
cm m
ol
l-1
0 3 6 9 12 15 18 210
2
4
6
[Q](mM)
0/
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30
863
864
865
866
867
868
869
870
871
872
873
Fig. 6 The Plot of lnkq against free energy (G0) for Coumarin-DMA systems inside 874
Niosome. 875
876
877
878
879
880
881
882
883
884
885
886
887
888
-0.8 -0.6 -0.4 -0.2
23.5
24.0
24.5
25.0
25.5
G0
lnk
q
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31
889
Table of content only: 890
891
892
893
894
895
896
897
Electron Transfer study inside bio-mimicking organized media like Niosome 898
helps to understand various chemical reactions occur in natural biological 899
systems. 900
901
902
903
904
905
906 907
908
909
910
911
Electronic Supplementary Material (ESI) for Physical Chemistry Chemical PhysicsThis journal is © The Owner Societies 2012