Interaction of fluorescence dyes with 5-fluorouracil: A photoinduced electron transfer study in bulk and biologically relevant water

Chemical Physics Letters 613 (2014) 115–121

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Interaction of fluorescence dyes with 5-fluorouracil: A photoinduced electron transfer study in bulk and biologically relevant water Jagannath Kuchlyan, Debasis Banik, Niloy Kundu, Arpita Roy, Nilmoni Sarkar ∗ Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, WB, India

a r t i c l e

i n f o

Article history: Received 21 July 2014 In final form 26 August 2014 Available online 1 September 2014

a b s t r a c t The interactions of widely used chemotherapeutic drug, 5-fluorouracil (5FU) with coumarin dyes have been investigated for the first time using steady-state and time-resolved fluorescence spectroscopic measurements. The fluorescence quenching along with the decrease in lifetimes of excited state of coumarin derivatives with gradual addition of 5FU is explained by photoinduced electron transfer (PET) mechanism. Our studies were performed in bulk water and confined water of AOT (aerosol OT) reverse micelle to investigate the effect of confinement on PET dynamics. The feasibility of PET reaction for coumarin-5FU systems is investigated calculating the standard free energy changes using the Rehm–Weller equation. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The 5-substituted pyrimidines belong to a class of base analogues having biological importance. Among them, 5FU is one of the oldest drugs used in chemotherapy, for the treatment of liver, lung, bladder, colon, skin, breast, pancreatic and head and neck cancers [1,2]. The antitumor activity of 5-FU is based on the inhibition of DNA synthesis by competitive inhibition of thymidylate synthetase, which is the target enzyme for the drug [1]. However, its clinical use has been restricted by its systemic toxicities [3]. Thus it is important to know the activity of 5FU in biologically relevant systems. Photoinduced electron transfer reaction between the base pairs of the DNA double helix explores ␲-stack-mediated electron transfer [4]. Strong stacking interactions between donor and acceptor result in fast electron-transfer kinetics as the close contact between donor and acceptor favours ET process. Moreover, Miranda et al. studied adsorption behaviour of 5FU on Au (III) surface and they proposed that ␲-stacking is enhanced when halogens are incorporated into the uracil structure [5]. It was also reported that uracil and its derivatives containing substituents ( F, Cl, Br, CH3 etc.) at 5-position have a significant ␲-electron charge density in the rings [6]. These investigations provide evidence for electron donating ability of 5FU in photoinduced electron transfer reaction. Recent literature reports on electron affinities, ionization energies, oxidation potential and protonation dynamics

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (N. Sarkar). http://dx.doi.org/10.1016/j.cplett.2014.08.062 0009-2614/© 2014 Elsevier B.V. All rights reserved.

of individual nucleic-acid bases have predicted the photoinduced interactions [7,8]. PET have been studied in detail by photophysical and photochemical behaviour of free fluorescence dyes of coumarin derivatives [9], acridine [10], rhodamine [11], 3,3 ,4,4 benzophenone tetracarboxylic acid [12], and oxazine [13]. In these cases the fluorescent dyes acted as electron acceptor in their excited state and nucleobases acted as electron donor in their ground state. Donor to acceptor photoinduced electron transfer (PET) is a very common reaction in chemistry and biology [9,10,14]. There are many reports regarding the investigation of photoinduced electron transfer reaction in reverse micelle [15], micelle [16], noisome [17], ionic liquid [18], cyclodextrins [19], Protein-surfactant complexes [20] etc. Water plays a vital role in many biological phenomena such as electron transfer and proton transfer. The confined water of AOT (aerosol OT, sodium dioctyl sulfosuccinate) reverse micelle resembles with biological water [21]. The surfactant, AOT, is well characterized and is commonly used for the preparation of reverse micelles as it can solubilize large amount of water. Aqueous AOT reverse micelles consist of water, surfactants, and nonpolar solvents with appropriate ratios. The inner water pool of AOT/heptane/water reverse micelles is surrounded by polar head groups of surfactant molecules oriented towards the water pool and the nonpolar tail parts pointed outside, towards bulk heptane [22]. Inside AOT reverse micelles, the radius of the water pool is approximately 2w0 Å, where w0 denotes the molar ratio of water and AOT surfactant [23]. Substantial amount of water is solubilized in AOT reverse micelles; w0 ranges from 0 to 70 for many systems [24]. Recent investigations have increased to understand the nature of confined water since, confined water shows

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different dynamics than bulk water [25,26]. The water pool inside the AOT reverse micelles has been studied extensively. The confined water has different properties than bulk water with respect to H-bonding, polarity, pH, microfluidity and viscosity [24–27]. Many chemical reactions including PET have been performed in reverse micelles [14,15,28]. Recently Vauthey et al. [29] have investigated solvent viscosity effect on PET dynamics in DMSO-glycerol mixture. They assumed that DMSO-glycerol mixture exhibits homogeneous viscosity and isoviscous with ionic liquid and other organized assemblies. However, in very recent work, Ghosh et al. [30] pointed out the inhomogeneity of viscosity in DMSO-glycerol mixture using a very simple but intelligent probe dependent solvation dynamics measurement. The general conclusion drawn by Vauthey et al. [29] is questionable with respect to the experimental results of Ghosh et al. [30]. Restricted environments can influence PET dynamics and it has been studied that the rates of the reactions are largely changed in reverse miceller media compared to those in homogeneous media. It has been shown that water pool in confined media has a vital role in determining the efficiency with which ET reactions occur [15]. The localized orientation of the substrates along with the charge at the interfaces are important factors governing the efficiency of many reactions in reverse micelles [14,15,28]. In the present study, the electron transfer process between coumarin derivatives and 5FU in reverse micelle has been studied. In detail, the steady-state and time resolved fluorescence quenching taking place in various coumarin derivatives with 5FU donor have been studied in bulk water and AOT/n-heptane/water reverse micelle at w0 = 60. Owing to the higher solubility of 5FU in aqueous media it is expected that 5FU will remain inside the water pool of reverse micelle. So to solubilize significant amount of 5FU in AOT reverse micelle, measurements have been carried out at w0 = 60. It has been shown that fluorescence quenching as well as decrease of lifetime of excited state coumarin dyes in presence of 5FU is driven by PET. The feasibility of PET has been shown by calculating standard free energy change of PET systems. As 5FU solubilizes in water phase of reverse micelle, so it would be interesting to study the nature of PET dynamics for our systems in bulk water and water pool of AOT reverse micelle. We also want to compare rate of ET dynamics in bulk water and confined water of reverse micelle. 2. Experimental 2.1. Materials used All the coumarin dyes were purchased from the Exciton (laser grade) and used as received. The 5-fluorouracil was purchased from Sigma–Aldrich and used without further purification. AOT was obtained from Sigma–Aldrich and was dried in vacuum for 12–13 h prior to use. n-Heptane, purchased from Spectrochem (India), was used without further purification. Doubly distilled deionised (MilliQ) water was used and added in appropriate quantity for the preparation of reverse micelle solutions having desired w0 value (w0 = 60). The AOT solutions were freshly prepared and the concentration was maintained at 0.09 M for all measurements. The chemical structures of the acceptors (all coumarin dyes), the donor (5FU), and AOT are given in Scheme 1.

of the time resolved fluorescence setup has been provided in our earlier publication [15]. For the excitation purpose in time resolved measurements, picosecond diode laser (IBH, Nanoled) was used and the signal collection was done at the magic angle of 54.71 degrees using a Hamamatsu microchannel plate photomultiplier tube (3809U). The instrument response function of our setup is 110 ps. Anisotropy measurements were done using the same setup. IBH DAS, version 6, and decay analysis software was used for data analysis. The anisotropy data was also analyzed using the same software. The cyclic voltametric (CV) measurements were performed on CH Instrument bipotentiostat (model CHI710D) in methanol medium using a glassy carbon electrode as working electrode. A platinum wire was used as the counter electrode and Ag/AgCl as the reference electrode. Tetrabutylammonium perchlorate (TBAP) was used as the supporting electrolyte. The instrumental values obtained were normalized with respect to saturated calomel electrode. The temperature was maintained at 298 ± 1 K for all other measurements. 3. Results and discussion 3.1. Steady state absorption and emission spectra The absorption and emission spectra of all coumarin dyes were measured in bulk water and in AOT reverse micelle at w0 = 60. The absorption spectrum of C-440 in AOT reverse micelle is similar to that in bulk water signifying that this molecule solubilizes preferentially in the water pool of reverse micelle. This is in accordance with our expectation because the free amino hydrogens of C-440 which can easily form intermolecular hydrogen-bonds with the water molecules and head groups of the surfactant. The absorption spectra of all other coumarin dyes show a distinct blue shift on moving from bulk water to AOT reverse micelle. This clearly indicates that these dye molecules reside primarily at the interior region (head group region) of the reverse micelles. It was observed that the absorption spectra of the coumarin dyes in AOT reverse micelle and bulk water remain unchanged upon addition of the 5FU in these solutions. This further suggests that no ground-state complex formation occurs between the coumarins and 5FU in the reverse micelle as well as bulk water. Similarly, in case of emission study, no significant blue shift is observed for C-440 on going from bulk water (emission maximum ∼444 nm) to AOT reverse micelle (emission maximum ∼440 nm). This observation provides an indication that the dye molecule resides inside the water pool of the AOT reverse micelle. The blue shift of the emission peaks for the other dyes in the reverse micelle are significant which indicates that the dye molecules mainly exist at the interfaces of reverse micelle. The absorption and emission peaks of all coumarin dyes in bulk water and AOT reverse micelle are listed in Table S1 (supporting information). 3.2. Time-resolved anisotropy measurements To locate the position of the probe, time-resolved fluorescence anisotropy study was performed in AOT reverse micelle and in bulk water. The time-resolved fluorescence anisotropy r(t) is given by the following equation I|| (t) − GI⊥ (t) I|| (t) + 2GI⊥ (t)

2.2. Instruments used

r(t) =

The absorption and fluorescence spectra were recorded using a Shimadzu (model no. UV-2450) spectrophotometer and a Hitachi (model no. F-7000) spectrofluorimeter, respectively. For steady state measurements Coumarin-440 (C-440) and Coumarin-450 (C-450) were excited at 375 nm; Coumarin-480 (C-480) and Coumarin-6H (C-6H) were excited at 408 nm. A detailed description

where I (t) and I⊥ (t) are the parallely and perpendicularly polarized fluorescence decays with respect to the polarization of the excitation light, respectively. G is the correction factor for detector sensitivity to the polarization direction of the emission. The value of G factor in our case is 0.60. The anisotropy decays and their fitted curves for all coumarins in both the systems are shown in

(1)

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H

H H

117

N

O

O

C2H5

N

O

O

H3C CH3

CH3

Coumarin-450 (C-450)

Coumarin-440 (C-440)

N

O

O

N

O

O

CH3 Coumarin-480 (C-480)

Coumarin-6H (C-6H)

O F

O

N

H

SO3- Na+

O O

N

O

O

H 5-Fluorouracil (5FU)

Sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (NaAOT)

Scheme 1. Chemical structures of coumarin derivatives, 5-fluorouracil and AOT.

Figure S1 (supporting information). The rotational relaxation times ( rot for single exponential,  fast and  slow for biexponential) and their relative contributions (afast and aslow ) as estimated from the analysis of the anisotropy decays are tabulated in Table S2 (supporting information). The  rot values for all coumarin dyes in homogeneous water are in the range of 90–140 ps while in AOT reverse micelle at w0 = 60 are in the range 520–660 ps. Thus large value of rotational relaxation time in AOT reverse micelle provides an indication to the existence of the dye molecules in the reverse miceller core and they experience a much more constrained environment in AOT reverse micelle compared to that in bulk water. 3.3. Quenching study using steady state and time resolved fluorescence measurements The fluorescence quenching of coumarin dyes by 5FU was investigated by steady-state and time-resolved fluorescence spectroscopy in AOT reverse micelle at w0 = 60 and bulk water. The nature and shape of the emission spectra of all coumarin dyes remain same even with the maximum concentration of 5FU. Hence, the possibility of any excited-state complex formation is ruled out. From steady-state (SS) and time-resolved (TR) measurements, significant quenching of fluorescence intensity and decrease of lifetime of coumarin dyes was observed for both reverse micelle and bulk water on addition of the 5FU. The SS quenching of fluorescence dyes are shown in Figures 1 and 2 and Figure S2 (Supporting information) and the TR quenching of these dyes are shown

in Figures 3 and 4 and Figure S3 (supporting information). The fluorescence-quenching constant is given by Stern–Volmer equation: Io 0 = 1 + KSV [Q ] = 1 + kq 0 [Q ] = I 

(2)

where I0 and I denote steady state fluorescence intensities, and  0 and  are the fluorescence lifetimes of the coumarin dyes in the absence and presence of the quencher, respectively. KSV is the Stern–Volmer constant, and [Q] is the quencher concentration. The plot for  0 / vs [Q] for various coumarin molecules with increasing concentration of 5FU in bulk water as well as in AOT reverse micelle are shown in Figure 5 and Figure S4 (supporting information). The time-resolved fluorescence decays were fitted by bi-exponential function for reverse micelle and single exponential for bulk water. The bi-exponential decays are originating in AOT reverse micelle because the probe molecules face heterogeneous environment in the reverse micelle. The following equation was used to calculate lifetime by averaging the component: av  =  = a1 1 + a2 2

(3)

where  1 and  2 are the two components of fluorescence lifetime, and a1 and a2 are their relative amplitudes. The parameters of fluorescence decay of the dye molecules in homogeneous aqueous solution and reverse micelle are summarized in the Tables 1 and 2, respectively. The ET rate constant (kq ) values for coumarins-5FU in bulk water and reverse micelle are listed in Tables 1 and 3.

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Bulk water

(i)

2500

2500

Fl.Intensity (a.u.)

3000

Fl. Intensity (a.u.)

3000

(a)

AOT at W0=60

3500

(vi)

2000 1500 1000

(b) (i)

2000

(vi)

1500 1000 500

500 400

420

440

460

480

500

520

0

540

Wavelength (nm)

400

420

440

460

480

500

520

540

Wavelength (nm)

Figure 1. Steady state fluorescence quenching of C-440 in (a) AOT at w0 = 60. (b) Bulk water with gradual addition of 5FU from 0 mM to 9.61 mM.

AOT at W0=60

2500

(a)

(i)

1000 800

(vi)

600 400

Bulk water

2000

Fl. Intensity (a.u.)

Fl. Intensity (a.u.)

1200

(b)

(i)

1500

(vi) 1000 500

200 400

450

500

550

440

Wavelength (nm)

480

520

560

Wavelength (nm)

Figure 2. Steady state fluorescence quenching of C-450 in (a) AOT at w0 = 60. (b) Bulk water with gradual addition of 5FU from 0 mM to 9.61 mM.

The rate constants of PET (kq ) were determined using Eq. (2). The highest rate constant of PET was obtained in case of C-440 in bulk water (∼4.84 × 109 M−1 s−1 ). Similarly in AOT reverse micelle, C-440 shows grater ET rate constant (∼10.91 × 109 M−1 s−1 ) compared to other coumarin dyes. This is due to the strong H-bonding interaction between C-440 and 5FU in both bulk water and AOT

reverse micelle. C-440 has a free NH2 group so in the excited state it forms strong hydrogen bonds with 5FU. The hydrogen bonding facilitates the close contact of donor–acceptor pairs and tight packing that leads to strong electronic coupling resulting in fast electron transfer [31]. Again H-bonding stabilizes the activated complexes and decreases the free activation energy for the ET reaction. As a

(a) (i)

(iv)

100

(i)

1000

Counts

1000

counts

(b)

10

(iv)

100

10

3

6

9

12

15

18

Time (ns)

21

24

27

30

5

10

15

20

25

30

35

Time (ns)

Figure 3. Time-resolved fluorescence decays of C-440 in (a) AOT at w0 = 60. (b) Bulk water with gradual addition of 5FU from 0 mM to 9.61 mM.

J. Kuchlyan et al. / Chemical Physics Letters 613 (2014) 115–121

119

(b)

(a) (i)

(iv)

100

(i)

1000

Counts

Counts

1000

(iv)

100

10

10 0

10

20

0

30

10

20

30

Time (ns)

Time (ns)

Figure 4. Time-resolved fluorescence decays of C-450 in (a) AOT at w0 = 60. (b) Bulk water with gradual addition of 5FU from 0 mM to 9.61 mM.

result, strong H-bonding speeds up the ET rate and hence combination of C-440 and 5FU shows faster electron transfer rate [32]. Moreover, steady state absorption spectra show that C-440 resides predominantly in the water pool of reverse micelle compared to other coumarin dyes and due to higher aqueous solubility of 5FU; it also resides in the water pool of reverse micelle. As a result, extent of quenching of C-440 by 5FU is greater than other coumarins in AOT reverse micelle. In case of other coumarin dyes, there occurs partitioning of the probes between water pool of reverse micelle and interfacial region of reverse micelle. PET can be happened between 5FU and this significant amount of dye molecules which reside in water pool of reverse micelle. Physicochemical properties of 5FU have been studied by Gustavsson et al. and they have provided the information of strong H-bonding ability of 5FU [33–35]. Moreover, strong H-bonding and electron donating ability of 5FU also be explained by considering keto-enol tautomerism. Theoretically and experimentally, many research groups investigated several tautomeric forms of 5FU and also gave the information of anions formation in water [36,37]. The enol form of 5FU possesses a benzenoid structure with an aromatic sextet of electrons. An aromatic benzenoid nucleus has higher electron density than a nonbenzeniod one [38]. So the enol form is expected to be more prone towards ET and is also having strong H-bonding ability with the coumarin dyes.

1.4

(a)

1.4

(b)

1.3

1.2

τ 0 /τ

τ 0/τ

At present, it is interesting to compare the ET rate constant values in AOT reverse micelle with those in bulk water. The ET rate constant (Kq ) values are observed higher in AOT reverse micelle compared to bulk water for all coumarin-5FU systems. This may be understood by considering the close proximity and strong stacking interaction between coumarin dyes and 5FU in restricted environment compared to homogeneous aqueous medium. It is previously mentioned that ET occurs when donor–acceptor are in close contact and strong stacking interaction between them [4]. Moreover, ET in AOT reverse micelle occurs under nondiffusive condition but in bulk water diffusive condition. It is reported that the diffusion of reactants does not influence quenching kinetics in AOT reverse micelle significantly; the quenching kinetics was attributed to the close proximity of donor and acceptor [17]. Nad and Pal compared ET dynamics under diffusive and nondiffusive condition and indicated that ET dynamics is faster in the latter case than in the former [39]. Due to incessant motion of reactants in homogeneous solution the close proximity of donor–acceptor in a particular time is rare. While in AOT reverse micelle, reactants are confined that reduces the random distribution and results in a controlled study of strong interaction within the molecules. The strong H-bonding interaction between coumarin dyes and 5FU in AOT reverse micelle give rise to higher ET rate compared to homogeneous solution having such weak interactions. Moreover, the strength of H-bonding of

1.2 1.1

1.0

0.8

1.0 0.9

0

2

4

6 5FU (mM)

8

10

0

2

4

6

8

5FU (mM)

Figure 5. Stern–Volmer plots for  0 / vs [5FU] of (a) C-440 and (b) C-450 () AOT at w0 = 60 and () bulk water.

10

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Table 1 Lifetimes of coumarin derivatives in bulk water as well as the corresponding bimolecular quenching constants as obtained from time-resolved fluorescence quenching study for 5FU. Acceptors

[5FU] (mM)

a (ns)

kq b (109 M−1 s−1 )

C-440

0 1.92 3.84 5.76 7.69 9.61

4.92 4.74 4.55 4.35 4.18 4.01

4.84

C-450

0 1.92 3.84 5.76 7.69 9.61

4.76 4.57 4.42 4.23 4.08 3.95

4.46

C-480

0 1.92 3.84 5.76 7.69 9.61

5.72 5.56 5.24 5.03 4.83 4.67

4.16

C-6H

0 1.92 3.84 5.76 7.69 9.61

5.12 4.98 4.79 4.56 4.41 4.22

4.33

a b

Acceptors

[5FU] (mM)

 1 (a1 ) (ns)

 2 (a2 ) (ns)

< av >a (ns)

C-440

0 1.92 3.84 5.76 7.69 9.61

0.89 (0.04) 0.71 (0.06) 0.59 (0.07) 0.59 (0.11) 0.67 (0.12) 0.53 (0.14)

4.21(0.96) 3.90 (0.94) 3.71 (0.93) 3.50 (0.89) 3.37 (0.88) 3.21 (0.86)

4.08 3.71 3.49 3.18 3.05 2.83

C-450

0 1.92 3.84 5.76 7.69 9.61

2.10 (0.01) 1.90 (0.01) 1.84 (0.02) 1.76 (0.04) 1.70 (0.05) 1.48 (0.09)

4.10 (0.99) 3.89 (0.99) 3.63 (0.98) 3.48 (0.96) 3.37 (0.95) 3.23(0.91)

4.08 3.87 3.59 3.41 3.29 3.07

C-480

0 1.92 3.84 5.76 7.69 9.61

0.89 (0.03) 0.57 (0.06) 0.66 (0.07) 0.62 (0.10) 0.57 (0.13) 0.49 (0.16)

5.17(0.97) 4.93 (0.94) 4.69 (0.93) 4.46 (0.90) 4.25 (0.87) 4.05 (0.84)

5.04 4.67 4.41 4.08 3.77 3.48

C-6H

0 1.92 3.84 5.76 7.69 9.61

2.49 (0.02) 2.33 (0.04) 2.19 (0.07) 1.99 (0.10) 1.90 (0.13) 1.85 (0.15)

5.11 (0.98) 4.85 (0.96) 4.59 (0.93) 4.39 (0.90) 4.23 (0.87) 4.13 (0.85)

5.06 4.75 4.42 4.15 3.93 3.79

a

Error in experimental data ±5%. Error in experimental data ±2%.

water inside the AOT reverse micelle is stronger than homogeneous water [24]. Recently the studies on excited-state intermolecular hydrogen bonding dynamics gave information on intermolecular PET and it was suggested that PET processes occur at faster rate in the presence of hydrogen bonding solvents compared to that in non-hydrogen bonding solvents [40,41]. Finally, for a quantitative understanding on ET, it is essential to calculate the free energy change (G0 ) for each coumarin-5FU system as rate of ET depends on free energy change. G0 is usually calculated using the following Rehm–Weller equation [42]. G0 = E(D\D+ ) − E(A\A− ) − E00 −

Table 2 Lifetimes and fluorescence decay parameters of coumarin derivatives in AOT reverse micelle at w0 = 60 for 5FU.

e2 4˘ε0 r

(4)

where E(D/D+ ) and E(A/A− ) are the oxidation potential of the donor and reduction potential of the acceptor, respectively. We have observed two peaks (oxidation and reduction) in our CV waves of 5FU and coumarin dyes in methanol. Because of the similar polarity of the reverse micelle and methanol, all the oxidation and reduction potential measurements have been accomplished in methanol using CV measurement. Here, E00 denotes the amount of energy required by the coumarin dyes for transition from the ground state (S0 ) to the first excited electronic state (S1 ), which has been determined from the point of intersection of normalized absorption and emission spectra. In this equation, ‘e’ is the charge of the

Error in experimental data ±5%.

electron and ε0 is the static dielectric constant of the medium. For the calculation of free energy change, we have used the value of dielectric constant of 60% methanol as the emission maxima of the coumarin dyes become almost similar to that of the emission maxima in 60% methanol. We have chosen dielectric constant of 60% methanol solution ∼43.7 at 250 c. Here ‘r’ denotes the distance between the donor and the acceptor and is considered to be equal to the sum of the radii of the donor and the acceptor. Edward’s volume addition method was followed to estimate the radii of the donor and acceptor molecules, assuming the molecules to be spherical [43]. We have calculated all the ET-related parameters and G0 values in our present systems and are summarized in Table 3. Sedel et al. [9] also determined the redox potentials of nucleobases and coumarin derivatives in organic solvent using CV measurements and the feasibility of PET between them was investigated by calculating standard free energy change (G0 ) using Rehm–Weller equation [42]. From the Table 3, it is clear that C-440 and C-450 have higher G0 values than other two dye molecules. This may be ascribed due to strong H-bonding interaction of free NH2 and NH groups of C-440 and C450 respectively with 5FU. The dependence of electron transfer on driving force in a series of hydrogen bonded donor and acceptor systems have been reported by Gopidas et al. [44,45]. Several research groups also investigated free energy dependent photoinduced electron transfer for various donors and acceptors pairs and it is shown that PET process follows Rehm–Weller behaviours [44–48].

Table 3 Time-resolved quenching constants, ground-state redox potentials of donor and acceptors, radii of donor and acceptors, E00 values and G0 for different coumarin-5FU systems in AOT reverse micelle at w0 = 60. Systems

E00 (eV)

EA+/A V/(Vs) SCE

ED/D+ V/(Vs) SCE

RA (A0 )

C-440 C-450 C-480 C-6H

3.12 3.01 2.91 2.85

−1.08 −0.86 −1.07 −1.09

1.04

3.31 3.54 3.82 3.74

a

Error in experimental data ±2%.

RD (A0 ) 2.84

G0 a (eV) −1.05 −1.16 −0.85 −0.77

kq a (109 M−1 s−1 ) 10.91 8.34 9.11 7.12

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4. Conclusion Photoexcited interaction between coumarin dye molecules and 5FU have been successfully investigated in two types of media using steady-state and time resolved measurements. In heterogeneous AOT reverse micelle medium, molecules are confined in water pool, while in homogeneous medium they are free to move. In both media, fluorescence of coumarin dyes was substantially quenched along with the decrease in lifetime on addition of 5FU and the mechanism of quenching is governed by PET. Change in medium plays an important role in affecting the PET dynamics. The observed electron transfer rate constants have been found to be faster in AOT reverse micelle compared that of homogeneous bulk water. The feasibility of PET is confirmed by calculating standard free energy changes according to Rehm–Weller equation. From this conclusion we can say that PET plays an important role in the quenching mechanism of the present systems. Acknowledgments N.S. thanks Department of Science and Technology (DST) and Council of Scientific and Industrial Research (CSIR), Government of India, for generous research grants. J.K. is thankful to UGC and A.R. is thankful to CSIR for their research fellowships. N.K and D.B. are thankful to IIT-Kharagpur for their research fellowships. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2014.08.062. References [1] Fluorouracil: American hospital formulacy service drug information, in: G.K. McEvoy (Ed.), American Society of Hospital Pharmacists, Bethesda, Maryland, 1993. [2] B. Chabner (Ed.), Pharmacologic Principles of Cancer Treatment, Saunders, Philadelphia, 1982. [3] Y.-C. He, J.-W. Chen, J. Cao, D.-Y. Pan, J.-G. Qiao, World J. Gastroenterol. 9 (2003) 1795. [4] S.O. Kelley, J.K. Barton, Science 283 (1999) 375. [5] H.B. Aguiar, F.G.C. Cunha, F.C. Nart, P.B. Miranda, J. Phys. Chem. C 114 (2010) 6663. [6] M.J.G. Moa, R.A. Mosquera, J. Phys. Chem. A 110 (2006) 5934. [7] B.T. Psciuk, R.L. Lord, B.H. Munk, H.B. Schlegel, J. Chem. Theory Comput. 8 (2012) 5107.

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