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

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

2

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

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

3

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

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

4

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 Marc