SOLVATOCHROMIC STUDIES OF FLUORESCEIN DIANION IN N,N-DIMETHYLFORMAMIDE WATER AND DIMETHYLSULFOXIDE WATER

The absorption, excitation and fluorescence spectra of the fluorescein dianion (FL(2-)) in N,N-Dimethylformamide (DMF)/water (H2O) and Dimethylsulphoxide (DMSO)/H2O solvent mixtures have been investigated. It is found that the absorption lambda(max) and emission maxima EX(max) are both hypsochromic shifted when the H2O content in the solvent mixtures increases. However, the shoulder peaks lambda(s) remain constant at 483.5nm within the range of H2O mole fraction, x = 0 to 0.518 in a DMF/H2O solvent mixture and at 484.4nm within the range, x = 0 to 0.304, in a DMSO/H2O mixture. Further increases in H2O content, cause a hypsochromic shift in lambda(s). The molar energies for the electronic transition from the ground state (S-0) to S-1, the first excited singlet state, (E(T)(1)) and to S-2, the 2nd excited singlet state, (E(T)(2)) of FL(2-) in the solvent mixtures are also plotted against x. Linear straight lines and intersection points are observed at x = 0.73 for E(T)(1) and at x = 0.51 for E(T)(2) in the DMF/H2O mixture and at x = 0.71 for E(T)(1) and at x = 0.31 & 0.69 for E(T)(2) in the DMSO/H2O mixture. Hydrogen-bonding stabilization effects are used to explain the above observations. The variation in relative fluorescence quantum yields of S-1 and S-2 of FL(2) with x in the aprotic solvent-H2O mixtures to FL(2-) in pure aprotic solvent are determined and discussed.