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

Abstract

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