Energy Transfer Studies in Binary Dye Solution Mixture of Coumarin 440 + Coumarin 540 and its

The sensitized fluorescence emission from the bimixture [Coumarin 440 (donor) + Coumarin 540 (acceptor)] has been measured as a function of dye concentration using a fluorescence spectrophotometer. The concentration of Coumarin 440 (donor) is kept constant at 5×10-5M while the concentration of Coumarin 540 (acceptor) is varied. The spectroscopic parameters for this bimixture have been calculated from the concentration dependence of peak fluorescence intensity curves. Also the data obtained from the Horiba JobinYvon Time Resolved Fluorescence Spectrophotometer was analyzed for the determination of lifetimes at various emission peaks. A comparative study between theoretically calculated lifetimes and experimentally obtained lifetimes shows a good agreement. The results indicate that the energy transfer process between unlike molecules can be studied by lifetime measurements (which could also be determined from the fluorescence emission spectral studies). Many attempts have been made in studies aimed to improve dye lasers efficiency and to extend their spectral range of operation. Energy transfer dye lasers (ETDL) have generated considerable interest as they are useful in obtaining enhanced laser output, wide tuning range, reduced concentration and pump threshold requirements and ultra-short pulse width in contrast to their single dye counterpart [1-7]. Energy Transfer Dye Lasers using numerous donor-acceptor dye pairs have been reported by various investigators duringthe last five decades. Fluorescence energy transfer is the transfer of the excited state energy from a donor (D) to acceptor (A) [8–19]. This transferoccurs without the appearance of photon and is primarily a resultof dipole–dipole interaction between the donor and the acceptor.The rate of energy transfer depends upon the extent of overlapof the emission spectrum of the donor with the absorptionspectrum of the acceptor, the relative orientation of the donorand acceptor transition dipoles and the distance between thesemolecules. The non-radiative energy transfer occurs as a result ofdipole–dipole coupling between the donor and the acceptor,anddoes not involve the emission and reabsorption of photons. In contrast to these trivial factorsnonradiative energy transfer depends upon the molecular detailsof donor–acceptor pairs.Non-radiative energy transfer is effective overdistance ranging of 50A°.The intervening of solvent or other macromoleculeshas little effect on the efficiency of the energy transfer, whichdepends primarily on the D–A distances[18].Dye lasers have some limitations as the dye solution used as an active medium absorbs energy from the excitation source in a very limited range and so the emission band also has these limitations. If a dye laser has to be used as an ideal source its spectral region needs to be extended. In order to extend the spectral region of operation mixtures of different dye solutions/dye molecules embedded in solid matrices arebeingused.The use of such energy transfer in dye lasers is also helpful in minimizing the photoquenching effects and thereby, increasing the laser efficiency. In the present studies the energy transfer mechanism has been investigated in the ethanol solution mixture, i.e. Coumarin 440 and Coumarin 540 from their absorption and emission spectra. The dependence of lifetimes of the dye molecules on their


A B S T R A C T
The sensitized fluorescence emission from the bimixture [Coumarin 440 (donor) + Coumarin 540 (acceptor)] has been measured as a function of dye concentration using a fluorescence spectrophotometer.The concentration of Coumarin 440 (donor) is kept constant at 5×10 -5 M while the concentration of Coumarin 540 (acceptor) is varied.The spectroscopic parameters for this bimixture have been calculated from the concentration dependence of peak fluorescence intensity curves.Also the data obtained from the Horiba JobinYvon Time Resolved Fluorescence Spectrophotometer was analyzed for the determination of lifetimes at various emission peaks.A comparative study between theoretically calculated lifetimes and experimentally obtained lifetimes shows a good agreement.The results indicate that the energy transfer process between unlike molecules can be studied by lifetime measurements (which could also be determined from the fluorescence emission spectral studies).
Many attempts have been made in studies aimed to improve dye lasers efficiency and to extend their spectral range of operation.Energy transfer dye lasers (ETDL) have generated considerable interest as they are useful in obtaining enhanced laser output, wide tuning range, reduced concentration and pump threshold requirements and ultra-short pulse width in contrast to their single dye counterpart [1][2][3][4][5][6][7].Energy Transfer Dye Lasers using numerous donor-acceptor dye pairs have been reported by various investigators duringthe last five decades.Fluorescence energy transfer is the transfer of the excited state energy from a donor (D) to acceptor (A) [8][9][10][11][12][13][14][15][16][17][18][19].This transferoccurs without the appearance of photon and is primarily a resultof dipole-dipole interaction between the donor and the acceptor.The rate of energy transfer depends upon the extent of overlapof the emission spectrum of the donor with the absorptionspectrum of the acceptor, the relative orientation of the donorand acceptor transition dipoles and the distance between thesemolecules.The non-radiative energy transfer occurs as a result ofdipole-dipole coupling between the donor and the acceptor,anddoes not involve the emission and reabsorption of photons.In contrast to these trivial factorsnonradiative energy transfer depends upon the molecular detailsof donor-acceptor pairs.Non-radiative energy transfer is effective overdistance ranging of 50A°.The intervening of solvent or other macromoleculeshas little effect on the efficiency of the energy transfer, whichdepends primarily on the D-A distances [18].Dye lasers have some limitations as the dye solution used as an active medium absorbs energy from the excitation source in a very limited range and so the emission band also has these limitations.If a dye laser has to be used as an ideal source its spectral region needs to be extended.In order to extend the spectral region of operation mixtures of different dye solutions/dye molecules embedded in solid matrices arebeingused.The use of such energy transfer in dye lasers is also helpful in minimizing the photoquenching effects and thereby, increasing the laser efficiency.In the present studies the energy transfer mechanism has been investigated in the ethanol solution mixture, i.e.Coumarin 440 and Coumarin 540 from their absorption and emission spectra.The dependence of lifetimes of the dye molecules on their concentrations in ethanol solution has beenstudied and the results are found to be in good agreement as reported in the literature.

Experimental:
Coumarin 440, Coumarin 540 dyes were procured from Sigma Chemicals (USA).They were used as received without further purification.Samples of various concentrations were prepared in Ethanol (spectroscopic grade, procured from Merck India Ltd.).
The concentrations of the dyes were varied in the range of 10 −3 M to 10 −6 M. For preparation of 10 -3 M solution of Coumarin 440, 8.7595 mg was added to 50 ml of Ethanol.And the solutions of concentrations 5×10 -4 M, 10 -4 M, 5×10 -5 M, 10 -5 M and 10 -6 M were prepared using 10 -3 M stock solution.Similarly, the other dye solutions were prepared.
For the energy transfer mechanism in the binary dye mixture solution, Coumarin 440 plus Coumarin 540, the donor ( Coumarin 440 ) concentration was fixed at 5×10 −5 M as it was found to be most efficient at this concentration.
The absorption spectra were recorded using Shimadzu (260) UV-Visible spectrophotometer.Cary Eclipse Spectrophoto-fluorometer (Varian make), was used to record the fluorescence emission spectra of the dye solutions and their mixtures under investigation.
The lifetimes for the various concentrations of the individual and binary dye mixture solutions were recorded using Horiba JobinYvon Time Resolved Fluorescence Spectrophotometer

Results & discussions: Excitation (absorption) and Emission spectra Overlaps:
The selection of dyes for this binary combination in the study was done such that the emission spectrum of the first dye (donor) overlaps the absorption spectra the other ones (acceptors).The extent of overlap decides the efficiency of energy transfer.The overlapping of the excitation (absorption) and emission spectra of bimixture under investigation are as shown in the figure 1 The graph shown in Figure 1 shows the excitation and emission spectra of binary dye mixture i.eCoumarin 440 plus Coumarin 540.The excitation plot of Coumarin 440 is taken at emission wavelength 428nm and emission plot is taken at excitation of 378nm.Similarly, the excitation plot of Coumarin 540 is taken at emission wavelength 501nm and emission plot is taken at excitation of 463nm.

The Excitation and Fluorescence Emission Spectra of Individual dyes and of binary dye mixture
The excitation and fluorescence emission spectra are recorded using Cary Eclipse Spectro-photo-fluorometer (Varian make) of individual dyes are represented in Figure 2 &     The excitation (absorption) wavelength was kept at 378 nm so that it does not get practically absorbed by Coumarin 540 dye and the energy emitted by Coumarin 440 only is absorbed (accepted) by the acceptor dyes to get their characteristic emissions.The successive quenching of the Coumarin 440 emission is accompanied by enhancement in the intensity of the characteristic emissions of the acceptors (Coumarin 540).It could therefore, be clearly seen in these spectra that there is an energy transfer from the donor (Coumarin 440) to the acceptor (Coumarin 540).The observed blue shifts in the donor emission spectra with increasing acceptor concentrations, in cases, are due to radiative energy transfer and the red shift in the acceptor emission spectrum could be radiative migration amongst the acceptor molecules [43][44][45][46][47].

Gaussian Fits and Error Plots:
Comparison of experimental and theoretically fitted spectra by the Gaussian spectral convolution-deconvolution method of typical fluorescence emission spectrum for various dye mixture solutions is performed.The Experimental emission spectrum, theoretically convoluted emission spectrum and different deconvoluted Gaussian peaks are shown separately.The Error plots represent the accuracy of the theoretically fitted spectra over the experimental In figure 5, experimental fluorescence emission spectra and theoretically convoluted spectra of binary dye mixture of Coumarin 540 and Coumarin 440 are shown.Also their deconvoluted Gaussian peaks and the error plot have been shown.To see whether the nature of the emission spectra has been changed on addition of the donor dye (Coumarin 440), the emission spectra were where I0 is the initial value of the intensity (baseline offset), I is the intensity at wavelength λ, A is the total area under the curve from the baseline, λc is the peak position (centre of the peak/'mean'), w 2 is sigma (the 'variance'), approximately 0.849 the width of the peak at half the intensity and w/2 is the 'standard deviation'.The deconvoluted peaks were then convoluted and overlapped on the experimental spectrum to see the quality of the fitting.The overlapping of the experimental and theoretically simulated peaks and the resulting residue plot shows a good quality fitting.All the other emission spectra of other dye mixture solutions were also fitted using the same method at different number of Gaussian peaks.The other fluorescence emission spectra of other dyes mixture solutions are shown as: After plotting the data obtained from spectrofluorometer in origin revealed that for different dye mixture solutions there were more than one peak unlike the single dye solution.To calculate the lifetime and to study the energy transfer these plots were deeply studied.Due to the presence of different dyes in mixtures the emission plots were unlike the individual plots i.e. due to the overlapping of each dye's emission spectra these peaks were obtained and hence they were affected by each other's presence and it resulted in increased broad band spectra, which is quite useful in various lasing applications like in optical sensing.Thus in order to study individual dye's effect for calculating the parameters needed for lifetime these plots was deconvoluted.Deconvolution helps in distinguishing all the different plots leading to multi peak plot and analyzing them separately on the basis of number ofdifferentiating the curves for each and every dye present in the solution.For the calculations each and every single peak deconvoluted plot was examined and all the various parameters like area under the curve, half bandwidth were calculated and used for calculating the lifetime.The overlapping effect represented by the overlapping integral was analyzed and its effect was taken under consideration.This overlapping was seen after deconvolution.The area under the curve was calculated after selecting the deconvoluted plot.This curve was set as active and integrated with respect to emission wavelength & half bandwidth was also noted down for the calculations.The different parameters were noted for the calculations of the natural lifetime and the fluorescence quantum yield.So by using these values a theoretical study of the lifetime was done.

Stern Volmer Plots
The non-radiative energy transfer rate constant has also been calculated from the Stem-Volmer plot by the following equations: To evaluate the energy transfer parameters, the measured lifetime is fitted in Stern-Volmer equation: and in terms of the relative emission intensities of the donor in the absence and presence of the acceptor it could be written as whereI0d is the initial intensity of the donor in the absence of the acceptor which gets reduced to Id in the presence of the acceptor, and are the donor lifetimes in the absence and in the presence of acceptor having concentration [A], respectively.0 .7 0 0 .7 5 0 .8 0 0 .8 5 0 .9 0 0 .9 5 6 .0

Lifetime Results (Experimental)
The data obtained from the Horiba JobinYvon Time Resolved Fluorescence Spectrophotometer was analyzed for the determination of lifetimes for the various concentrations and emission peaks of the individual and binary dye mixture solutions.
The values of experimental lifetimes obtained for individual dyes at various concentrations are tabulated in Table 1 below.

Figure 1 :
Figure 1: The plot showing overlapping between fluorescence emission of donor (Coumarin 440) and absorption of acceptor (Coumarin 540) in binary dye mixture in ethanol

Fluorescence
emission spectra of the Coumarin 440 dye solution in ethanol excited by 378 nm for different concentrations are shown in the figure 2. The excitation spectra of the Coumarin 440 dye solution (keeping emission at 428 nm) has also been shown in the Figure 2.

Figure 2 :
Figure 2: Excitation and fluorescence Emission spectra of Coumarin 440 for different concentration Coumarin540 Fluorescence emission spectra of the Coumarin 540 dye solution in ethanol excited by 463 nm for different concentrations are shown in the Figure 3.The excitation spectra of the Coumarin 540 dye solution (keeping emission at 501 nm) has also been shown in the Figure 3.

Figure 3
Figure 3 Excitation and fluorescence Emission spectra of Coumarin 540 for different concentration

Figure 4 :
Figure 4:Fluorescence emission spectra of binary dye mixture of Coumarin 440 and Coumarin 540.The concentration of Coumarin 440 (donor) is fixed at 5x10 -5 M while concentration of Coumarin 540 (acceptor) is varied

Figure 5 :
Figure 5: Experimental and Theoretical fitted spectra by the Gaussian spectral convolution-deconvolution method for a typical fluorescence emission spectrum of Coumarin 440+Coumarin 540 and its corresponding error plot

Figure 6 :
Figure 6: Stern-Volmer plot of I0d/Id versus concentration of acceptor for binary dye mixture of Coumarin 440 and Coumarin 540 Stern-Volmer curves, plotted with varying concentrations of the acceptor, for binary dye mixture is shown in fig.6.It could be easily seen that Stern-Volmer, I0d / Id versus concentration plots are linear in nature.The plot is drawn by varying the concentration of Coumarin 540(i.e.acceptor) and keeping concentration of Coumarin 440(i.e.donor) constant..The value of the energy transfer rate constant KT (KD→A ) hence calculated by the stern volmer plot was found to be 3.2925 M -1 .

Table 1 :
Experimental Lifetimes obtained for individual dyes at various concentrations