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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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17

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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18

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

Electronic Supplementary Material (ESI) for Physical Chemistry Chemical PhysicsThis journal is © The Owner Societies 2012

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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

Electronic Supplementary Material (ESI) for Physical Chemistry Chemical PhysicsThis journal is © The Owner Societies 2012

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

Electronic Supplementary Material (ESI) for Physical Chemistry Chemical PhysicsThis journal is © The Owner Societies 2012

29

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/

Electronic Supplementary Material (ESI) for Physical Chemistry Chemical PhysicsThis journal is © The Owner Societies 2012

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

Electronic Supplementary Material (ESI) for Physical Chemistry Chemical PhysicsThis journal is © The Owner Societies 2012

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


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